diff --git a/src/PhpSpreadsheet/Cell.php b/src/PhpSpreadsheet/Cell.php
index 48f0f721..0263853d 100644
--- a/src/PhpSpreadsheet/Cell.php
+++ b/src/PhpSpreadsheet/Cell.php
@@ -118,7 +118,7 @@ class Cell
      *
      * @throws Exception
      */
-    public function __construct($pValue = null, $pDataType = null, Worksheet $pSheet = null)
+    public function __construct($pValue, $pDataType, Worksheet $pSheet)
     {
         // Initialise cell value
         $this->value = $pValue;
@@ -990,12 +990,8 @@ class Cell
      *
      * @throws Exception
      */
-    public static function setValueBinder(Cell\IValueBinder $binder = null)
+    public static function setValueBinder(Cell\IValueBinder $binder)
     {
-        if ($binder === null) {
-            throw new Exception('A \\PhpOffice\\PhpSpreadsheet\\Cell\\IValueBinder is required for PhpSpreadsheet to function correctly.');
-        }
-
         self::$valueBinder = $binder;
     }
 
diff --git a/src/PhpSpreadsheet/Chart/Legend.php b/src/PhpSpreadsheet/Chart/Legend.php
index dea8610d..2abb28e2 100644
--- a/src/PhpSpreadsheet/Chart/Legend.php
+++ b/src/PhpSpreadsheet/Chart/Legend.php
@@ -73,8 +73,9 @@ class Legend
     /**
      * Create a new Legend.
      *
-     * @param mixed $position
-     * @param mixed $overlay
+     * @param string $position
+     * @param Layout|null $layout
+     * @param bool $overlay
      */
     public function __construct($position = self::POSITION_RIGHT, Layout $layout = null, $overlay = false)
     {
diff --git a/src/PhpSpreadsheet/Chart/PlotArea.php b/src/PhpSpreadsheet/Chart/PlotArea.php
index f3c4d9f2..5c16db84 100644
--- a/src/PhpSpreadsheet/Chart/PlotArea.php
+++ b/src/PhpSpreadsheet/Chart/PlotArea.php
@@ -43,7 +43,8 @@ class PlotArea
     /**
      * Create a new PlotArea.
      *
-     * @param mixed $plotSeries
+     * @param Layout|null $layout
+     * @param array $plotSeries
      */
     public function __construct(Layout $layout = null, array $plotSeries = [])
     {
diff --git a/src/PhpSpreadsheet/Chart/Title.php b/src/PhpSpreadsheet/Chart/Title.php
index 491f5ecb..88f1e59e 100644
--- a/src/PhpSpreadsheet/Chart/Title.php
+++ b/src/PhpSpreadsheet/Chart/Title.php
@@ -44,6 +44,7 @@ class Title
      * Create a new Title.
      *
      * @param null|mixed $caption
+     * @param null|Layout $layout
      */
     public function __construct($caption = null, Layout $layout = null)
     {
diff --git a/src/PhpSpreadsheet/HashTable.php b/src/PhpSpreadsheet/HashTable.php
index cda8c0d5..e8ac403b 100644
--- a/src/PhpSpreadsheet/HashTable.php
+++ b/src/PhpSpreadsheet/HashTable.php
@@ -81,7 +81,7 @@ class HashTable
      *
      * @throws Exception
      */
-    public function add(IComparable $pSource = null)
+    public function add(IComparable $pSource)
     {
         $hash = $pSource->getHashCode();
         if (!isset($this->items[$hash])) {
@@ -97,7 +97,7 @@ class HashTable
      *
      * @throws Exception
      */
-    public function remove(IComparable $pSource = null)
+    public function remove(IComparable $pSource)
     {
         $hash = $pSource->getHashCode();
         if (isset($this->items[$hash])) {
diff --git a/src/PhpSpreadsheet/NamedRange.php b/src/PhpSpreadsheet/NamedRange.php
index 35942ea5..b1783662 100644
--- a/src/PhpSpreadsheet/NamedRange.php
+++ b/src/PhpSpreadsheet/NamedRange.php
@@ -238,7 +238,7 @@ class NamedRange
      *
      * @return NamedRange
      */
-    public static function resolveRange($pNamedRange, Worksheet $pSheet = null)
+    public static function resolveRange($pNamedRange, Worksheet $pSheet)
     {
         return $pSheet->getParent()->getNamedRange($pNamedRange, $pSheet);
     }
diff --git a/src/PhpSpreadsheet/ReferenceHelper.php b/src/PhpSpreadsheet/ReferenceHelper.php
index 5de4d727..30eb9436 100644
--- a/src/PhpSpreadsheet/ReferenceHelper.php
+++ b/src/PhpSpreadsheet/ReferenceHelper.php
@@ -384,7 +384,7 @@ class ReferenceHelper
      *
      * @throws Exception
      */
-    public function insertNewBefore($pBefore = 'A1', $pNumCols = 0, $pNumRows = 0, Worksheet $pSheet = null)
+    public function insertNewBefore($pBefore, $pNumCols, $pNumRows, Worksheet $pSheet)
     {
         $remove = ($pNumCols < 0 || $pNumRows < 0);
         $allCoordinates = $pSheet->getCoordinates();
diff --git a/src/PhpSpreadsheet/RichText.php b/src/PhpSpreadsheet/RichText.php
index f6d56772..8c9dc2f2 100644
--- a/src/PhpSpreadsheet/RichText.php
+++ b/src/PhpSpreadsheet/RichText.php
@@ -68,7 +68,7 @@ class RichText implements IComparable
      *
      * @return RichText
      */
-    public function addText(RichText\ITextElement $pText = null)
+    public function addText(RichText\ITextElement $pText)
     {
         $this->richTextElements[] = $pText;
 
diff --git a/src/PhpSpreadsheet/Shared/Date.php b/src/PhpSpreadsheet/Shared/Date.php
index cc1638d9..a7409e38 100644
--- a/src/PhpSpreadsheet/Shared/Date.php
+++ b/src/PhpSpreadsheet/Shared/Date.php
@@ -251,7 +251,7 @@ class Date
      *
      * @return float MS Excel serialized date/time value
      */
-    public static function dateTimeToExcel(\DateTimeInterface $dateValue = null)
+    public static function dateTimeToExcel(\DateTimeInterface $dateValue)
     {
         return self::formattedPHPToExcel(
             $dateValue->format('Y'),
diff --git a/src/PhpSpreadsheet/Shared/Font.php b/src/PhpSpreadsheet/Shared/Font.php
index c305579c..befc2751 100644
--- a/src/PhpSpreadsheet/Shared/Font.php
+++ b/src/PhpSpreadsheet/Shared/Font.php
@@ -330,7 +330,7 @@ class Font
      *
      * @return int Text width in pixels (no padding added)
      */
-    public static function getTextWidthPixelsApprox($columnText, \PhpOffice\PhpSpreadsheet\Style\Font $font = null, $rotation = 0)
+    public static function getTextWidthPixelsApprox($columnText, \PhpOffice\PhpSpreadsheet\Style\Font $font, $rotation = 0)
     {
         $fontName = $font->getName();
         $fontSize = $font->getSize();
diff --git a/src/PhpSpreadsheet/Style/Border.php b/src/PhpSpreadsheet/Style/Border.php
index 48497aa5..9c2d8891 100644
--- a/src/PhpSpreadsheet/Style/Border.php
+++ b/src/PhpSpreadsheet/Style/Border.php
@@ -256,7 +256,7 @@ class Border extends Supervisor implements \PhpOffice\PhpSpreadsheet\IComparable
      *
      * @return Border
      */
-    public function setColor(Color $pValue = null)
+    public function setColor(Color $pValue)
     {
         // make sure parameter is a real color and not a supervisor
         $color = $pValue->getIsSupervisor() ? $pValue->getSharedComponent() : $pValue;
diff --git a/src/PhpSpreadsheet/Style/Fill.php b/src/PhpSpreadsheet/Style/Fill.php
index 57d64f34..9cbf6841 100644
--- a/src/PhpSpreadsheet/Style/Fill.php
+++ b/src/PhpSpreadsheet/Style/Fill.php
@@ -264,7 +264,7 @@ class Fill extends Supervisor implements \PhpOffice\PhpSpreadsheet\IComparable
      *
      * @return Fill
      */
-    public function setStartColor(Color $pValue = null)
+    public function setStartColor(Color $pValue)
     {
         // make sure parameter is a real color and not a supervisor
         $color = $pValue->getIsSupervisor() ? $pValue->getSharedComponent() : $pValue;
@@ -298,7 +298,7 @@ class Fill extends Supervisor implements \PhpOffice\PhpSpreadsheet\IComparable
      *
      * @return Fill
      */
-    public function setEndColor(Color $pValue = null)
+    public function setEndColor(Color $pValue)
     {
         // make sure parameter is a real color and not a supervisor
         $color = $pValue->getIsSupervisor() ? $pValue->getSharedComponent() : $pValue;
diff --git a/src/PhpSpreadsheet/Style/Font.php b/src/PhpSpreadsheet/Style/Font.php
index b40b788b..3436382a 100644
--- a/src/PhpSpreadsheet/Style/Font.php
+++ b/src/PhpSpreadsheet/Style/Font.php
@@ -528,7 +528,7 @@ class Font extends Supervisor implements \PhpOffice\PhpSpreadsheet\IComparable
      *
      * @return Font
      */
-    public function setColor(Color $pValue = null)
+    public function setColor(Color $pValue)
     {
         // make sure parameter is a real color and not a supervisor
         $color = $pValue->getIsSupervisor() ? $pValue->getSharedComponent() : $pValue;
diff --git a/src/PhpSpreadsheet/Worksheet.php b/src/PhpSpreadsheet/Worksheet.php
index 7565c854..4a6799d3 100644
--- a/src/PhpSpreadsheet/Worksheet.php
+++ b/src/PhpSpreadsheet/Worksheet.php
@@ -550,7 +550,7 @@ class Worksheet implements IComparable
      *
      * @return Chart
      */
-    public function addChart(Chart $pChart = null, $iChartIndex = null)
+    public function addChart(Chart $pChart, $iChartIndex = null)
     {
         $pChart->setWorksheet($this);
         if (is_null($iChartIndex)) {
@@ -1546,7 +1546,7 @@ class Worksheet implements IComparable
      *
      * @return Worksheet
      */
-    public function duplicateStyle(Style $pCellStyle = null, $pRange = '')
+    public function duplicateStyle(Style $pCellStyle, $pRange)
     {
         // make sure we have a real style and not supervisor
         $style = $pCellStyle->getIsSupervisor() ? $pCellStyle->getSharedComponent() : $pCellStyle;
@@ -1594,7 +1594,7 @@ class Worksheet implements IComparable
      *
      * @return Worksheet
      */
-    public function duplicateConditionalStyle(array $pCellStyle = null, $pRange = '')
+    public function duplicateConditionalStyle(array $pCellStyle, $pRange = '')
     {
         foreach ($pCellStyle as $cellStyle) {
             if (!($cellStyle instanceof Style\Conditional)) {
diff --git a/src/PhpSpreadsheet/Worksheet/HeaderFooter.php b/src/PhpSpreadsheet/Worksheet/HeaderFooter.php
index 6a0d084e..92760132 100644
--- a/src/PhpSpreadsheet/Worksheet/HeaderFooter.php
+++ b/src/PhpSpreadsheet/Worksheet/HeaderFooter.php
@@ -428,7 +428,7 @@ class HeaderFooter
      *
      * @return HeaderFooter
      */
-    public function addImage(HeaderFooterDrawing $image = null, $location = self::IMAGE_HEADER_LEFT)
+    public function addImage(HeaderFooterDrawing $image, $location = self::IMAGE_HEADER_LEFT)
     {
         $this->headerFooterImages[$location] = $image;
 
diff --git a/src/PhpSpreadsheet/Writer/Ods.php b/src/PhpSpreadsheet/Writer/Ods.php
index 4276fa6d..f82d4a73 100644
--- a/src/PhpSpreadsheet/Writer/Ods.php
+++ b/src/PhpSpreadsheet/Writer/Ods.php
@@ -48,7 +48,7 @@ class Ods extends BaseWriter implements IWriter
      *
      * @param \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet
      */
-    public function __construct(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function __construct(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         $this->setSpreadsheet($spreadsheet);
 
@@ -183,7 +183,7 @@ class Ods extends BaseWriter implements IWriter
      *
      * @return self
      */
-    public function setSpreadsheet(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function setSpreadsheet(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         $this->spreadSheet = $spreadsheet;
 
diff --git a/src/PhpSpreadsheet/Writer/Xls/Font.php b/src/PhpSpreadsheet/Writer/Xls/Font.php
index 5b4ad203..ba5c51bd 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Font.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Font.php
@@ -45,7 +45,7 @@ class Font
      *
      * @param \PhpOffice\PhpSpreadsheet\Style\Font $font
      */
-    public function __construct(\PhpOffice\PhpSpreadsheet\Style\Font $font = null)
+    public function __construct(\PhpOffice\PhpSpreadsheet\Style\Font $font)
     {
         $this->colorIndex = 0x7FFF;
         $this->font = $font;
diff --git a/src/PhpSpreadsheet/Writer/Xls/Xf.php b/src/PhpSpreadsheet/Writer/Xls/Xf.php
index 3b5af382..23f76f6a 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Xf.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Xf.php
@@ -136,7 +136,7 @@ class Xf
      *
      * @param \PhpOffice\PhpSpreadsheet\Style The XF format
      */
-    public function __construct(\PhpOffice\PhpSpreadsheet\Style $style = null)
+    public function __construct(\PhpOffice\PhpSpreadsheet\Style $style)
     {
         $this->isStyleXf = false;
         $this->fontIndex = 0;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx.php b/src/PhpSpreadsheet/Writer/Xlsx.php
index 0885f927..56f1c508 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx.php
@@ -111,7 +111,7 @@ class Xlsx extends BaseWriter implements IWriter
      *
      * @param \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet
      */
-    public function __construct(\PhpOffice\PhpSpreadsheet\Spreadsheet $spreadsheet = null)
+    public function __construct(\PhpOffice\PhpSpreadsheet\Spreadsheet $spreadsheet)
     {
         // Assign PhpSpreadsheet
         $this->setSpreadsheet($spreadsheet);
@@ -411,7 +411,7 @@ class Xlsx extends BaseWriter implements IWriter
      *
      * @return Xlsx
      */
-    public function setSpreadsheet(Spreadsheet $spreadsheet = null)
+    public function setSpreadsheet(Spreadsheet $spreadsheet)
     {
         $this->spreadSheet = $spreadsheet;
 
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Chart.php b/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
index b6396f3f..418688f7 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Chart.php
@@ -47,7 +47,7 @@ class Chart extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeChart(\PhpOffice\PhpSpreadsheet\Chart $pChart = null, $calculateCellValues = true)
+    public function writeChart(\PhpOffice\PhpSpreadsheet\Chart $pChart, $calculateCellValues = true)
     {
         $this->calculateCellValues = $calculateCellValues;
 
@@ -358,7 +358,7 @@ class Chart extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDataLabels($objWriter, Layout $chartLayout = null)
+    private function writeDataLabels(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, Layout $chartLayout = null)
     {
         $objWriter->startElement('c:dLbls');
 
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Comments.php b/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
index 72dba7ea..e630b797 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Comments.php
@@ -35,7 +35,7 @@ class Comments extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeComments(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet = null)
+    public function writeComments(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -94,7 +94,7 @@ class Comments extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeComment(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, $pCellReference = 'A1', \PhpOffice\PhpSpreadsheet\Comment $pComment = null, $pAuthors = null)
+    private function writeComment(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, $pCellReference, \PhpOffice\PhpSpreadsheet\Comment $pComment, array $pAuthors)
     {
         // comment
         $objWriter->startElement('comment');
@@ -118,7 +118,7 @@ class Comments extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeVMLComments(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet = null)
+    public function writeVMLComments(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -187,12 +187,12 @@ class Comments extends WriterPart
      * Write VML comment to XML format.
      *
      * @param \PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter XML Writer
-     * @param string $pCellReference Cell reference
+     * @param string $pCellReference Cell reference, eg: 'A1'
      * @param \PhpOffice\PhpSpreadsheet\Comment $pComment Comment
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeVMLComment(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, $pCellReference = 'A1', \PhpOffice\PhpSpreadsheet\Comment $pComment = null)
+    private function writeVMLComment(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, $pCellReference, \PhpOffice\PhpSpreadsheet\Comment $pComment)
     {
         // Metadata
         list($column, $row) = \PhpOffice\PhpSpreadsheet\Cell::coordinateFromString($pCellReference);
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php b/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
index 32c681d5..bf01e978 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/ContentTypes.php
@@ -36,7 +36,7 @@ class ContentTypes extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeContentTypes(\PhpOffice\PhpSpreadsheet\Spreadsheet $spreadsheet = null, $includeCharts = false)
+    public function writeContentTypes(\PhpOffice\PhpSpreadsheet\Spreadsheet $spreadsheet, $includeCharts = false)
     {
         // Create XML writer
         $objWriter = null;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php b/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
index 3cc78cbe..7eb9a06e 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/DocProps.php
@@ -35,7 +35,7 @@ class DocProps extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeDocPropsApp(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeDocPropsApp(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -134,7 +134,7 @@ class DocProps extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeDocPropsCore(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeDocPropsCore(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -203,7 +203,7 @@ class DocProps extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeDocPropsCustom(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeDocPropsCustom(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         $customPropertyList = $spreadsheet->getProperties()->getCustomProperties();
         if (empty($customPropertyList)) {
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php b/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
index 18fe9945..afd27f30 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Drawing.php
@@ -90,7 +90,7 @@ class Drawing extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    public function writeChart(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Chart $pChart = null, $pRelationId = -1)
+    public function writeChart(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Chart $pChart, $pRelationId = -1)
     {
         $tl = $pChart->getTopLeftPosition();
         $tl['colRow'] = \PhpOffice\PhpSpreadsheet\Cell::coordinateFromString($tl['cell']);
@@ -293,7 +293,7 @@ class Drawing extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeVMLHeaderFooterImages(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet = null)
+    public function writeVMLHeaderFooterImages(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -442,7 +442,7 @@ class Drawing extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeVMLHeaderFooterImage(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, $pReference = '', \PhpOffice\PhpSpreadsheet\Worksheet\HeaderFooterDrawing $pImage = null)
+    private function writeVMLHeaderFooterImage(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, $pReference, \PhpOffice\PhpSpreadsheet\Worksheet\HeaderFooterDrawing $pImage)
     {
         // Calculate object id
         preg_match('{(\d+)}', md5($pReference), $m);
@@ -485,7 +485,7 @@ class Drawing extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Worksheet\Drawing[] All drawings in PhpSpreadsheet
      */
-    public function allDrawings(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allDrawings(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of all drawings
         $aDrawings = [];
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Rels.php b/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
index 74a1823f..2d4ceef5 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
@@ -35,7 +35,7 @@ class Rels extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -110,7 +110,7 @@ class Rels extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeWorkbookRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeWorkbookRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -193,7 +193,7 @@ class Rels extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeWorksheetRelationships(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet = null, $pWorksheetId = 1, $includeCharts = false)
+    public function writeWorksheetRelationships(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet, $pWorksheetId = 1, $includeCharts = false)
     {
         // Create XML writer
         $objWriter = null;
@@ -353,7 +353,7 @@ class Rels extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeHeaderFooterDrawingRelationships(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet = null)
+    public function writeHeaderFooterDrawingRelationships(\PhpOffice\PhpSpreadsheet\Worksheet $pWorksheet)
     {
         // Create XML writer
         $objWriter = null;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php b/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
index d89a3dcd..da414a2f 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/RelsRibbon.php
@@ -35,7 +35,7 @@ class RelsRibbon extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeRibbonRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeRibbonRelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php b/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
index b25e6705..dbf4dfed 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/RelsVBA.php
@@ -35,7 +35,7 @@ class RelsVBA extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeVBARelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeVBARelationships(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
index 195572f3..4232445a 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
@@ -85,49 +85,46 @@ class StringTable extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeStringTable(array $pStringTable = null)
+    public function writeStringTable(array $pStringTable)
     {
-        if ($pStringTable !== null) {
-            // Create XML writer
-            $objWriter = null;
-            if ($this->getParentWriter()->getUseDiskCaching()) {
-                $objWriter = new \PhpOffice\PhpSpreadsheet\Shared\XMLWriter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
-            } else {
-                $objWriter = new \PhpOffice\PhpSpreadsheet\Shared\XMLWriter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter::STORAGE_MEMORY);
-            }
+        // Create XML writer
+        $objWriter = null;
+        if ($this->getParentWriter()->getUseDiskCaching()) {
+            $objWriter = new \PhpOffice\PhpSpreadsheet\Shared\XMLWriter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
+        } else {
+            $objWriter = new \PhpOffice\PhpSpreadsheet\Shared\XMLWriter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter::STORAGE_MEMORY);
+        }
 
-            // XML header
-            $objWriter->startDocument('1.0', 'UTF-8', 'yes');
+        // XML header
+        $objWriter->startDocument('1.0', 'UTF-8', 'yes');
 
-            // String table
-            $objWriter->startElement('sst');
-            $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
-            $objWriter->writeAttribute('uniqueCount', count($pStringTable));
+        // String table
+        $objWriter->startElement('sst');
+        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/spreadsheetml/2006/main');
+        $objWriter->writeAttribute('uniqueCount', count($pStringTable));
 
-            // Loop through string table
-            foreach ($pStringTable as $textElement) {
-                $objWriter->startElement('si');
+        // Loop through string table
+        foreach ($pStringTable as $textElement) {
+            $objWriter->startElement('si');
 
-                if (!$textElement instanceof \PhpOffice\PhpSpreadsheet\RichText) {
-                    $textToWrite = \PhpOffice\PhpSpreadsheet\Shared\StringHelper::controlCharacterPHP2OOXML($textElement);
-                    $objWriter->startElement('t');
-                    if ($textToWrite !== trim($textToWrite)) {
-                        $objWriter->writeAttribute('xml:space', 'preserve');
-                    }
-                    $objWriter->writeRawData($textToWrite);
-                    $objWriter->endElement();
-                } elseif ($textElement instanceof \PhpOffice\PhpSpreadsheet\RichText) {
-                    $this->writeRichText($objWriter, $textElement);
+            if (!$textElement instanceof \PhpOffice\PhpSpreadsheet\RichText) {
+                $textToWrite = \PhpOffice\PhpSpreadsheet\Shared\StringHelper::controlCharacterPHP2OOXML($textElement);
+                $objWriter->startElement('t');
+                if ($textToWrite !== trim($textToWrite)) {
+                    $objWriter->writeAttribute('xml:space', 'preserve');
                 }
-
+                $objWriter->writeRawData($textToWrite);
                 $objWriter->endElement();
+            } elseif ($textElement instanceof \PhpOffice\PhpSpreadsheet\RichText) {
+                $this->writeRichText($objWriter, $textElement);
             }
 
             $objWriter->endElement();
-
-            return $objWriter->getData();
         }
-        throw new \PhpOffice\PhpSpreadsheet\Writer\Exception('Invalid string table array passed.');
+
+        $objWriter->endElement();
+
+        return $objWriter->getData();
     }
 
     /**
@@ -139,7 +136,7 @@ class StringTable extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    public function writeRichText(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\RichText $pRichText = null, $prefix = null)
+    public function writeRichText(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\RichText $pRichText, $prefix = null)
     {
         if ($prefix !== null) {
             $prefix .= ':';
@@ -224,7 +221,7 @@ class StringTable extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    public function writeRichTextForCharts(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, $pRichText = null, $prefix = null)
+    public function writeRichTextForCharts(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, $pRichText = null, $prefix = null)
     {
         if (!$pRichText instanceof \PhpOffice\PhpSpreadsheet\RichText) {
             $textRun = $pRichText;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Style.php b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
index 3be4b76c..96457aa1 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Style.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
@@ -35,7 +35,7 @@ class Style extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
@@ -166,7 +166,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill = null)
+    private function writeFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill)
     {
         // Check if this is a pattern type or gradient type
         if ($pFill->getFillType() === \PhpOffice\PhpSpreadsheet\Style\Fill::FILL_GRADIENT_LINEAR ||
@@ -187,7 +187,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeGradientFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill = null)
+    private function writeGradientFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill)
     {
         // fill
         $objWriter->startElement('fill');
@@ -232,7 +232,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writePatternFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill = null)
+    private function writePatternFill(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\Fill $pFill)
     {
         // fill
         $objWriter->startElement('fill');
@@ -271,7 +271,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeFont(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\Font $pFont = null)
+    private function writeFont(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\Font $pFont)
     {
         // font
         $objWriter->startElement('font');
@@ -352,7 +352,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeBorder(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\Borders $pBorders = null)
+    private function writeBorder(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\Borders $pBorders)
     {
         // Write border
         $objWriter->startElement('border');
@@ -390,7 +390,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeCellStyleXf(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style $pStyle = null, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    private function writeCellStyleXf(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style $pStyle, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // xf
         $objWriter->startElement('xf');
@@ -466,7 +466,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeCellStyleDxf(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style $pStyle = null)
+    private function writeCellStyleDxf(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style $pStyle)
     {
         // dxf
         $objWriter->startElement('dxf');
@@ -532,7 +532,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeBorderPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, $pName = 'left', \PhpOffice\PhpSpreadsheet\Style\Border $pBorder = null)
+    private function writeBorderPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, $pName, \PhpOffice\PhpSpreadsheet\Style\Border $pBorder)
     {
         // Write BorderPr
         if ($pBorder->getBorderStyle() != \PhpOffice\PhpSpreadsheet\Style\Border::BORDER_NONE) {
@@ -557,7 +557,7 @@ class Style extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeNumFmt(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Style\NumberFormat $pNumberFormat = null, $pId = 0)
+    private function writeNumFmt(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Style\NumberFormat $pNumberFormat, $pId = 0)
     {
         // Translate formatcode
         $formatCode = $pNumberFormat->getFormatCode();
@@ -580,7 +580,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style[] All styles in PhpSpreadsheet
      */
-    public function allStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         return $spreadsheet->getCellXfCollection();
     }
@@ -594,7 +594,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style\Conditional[] All conditional styles in PhpSpreadsheet
      */
-    public function allConditionalStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allConditionalStyles(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of all styles
         $aStyles = [];
@@ -620,7 +620,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style\Fill[] All fills in PhpSpreadsheet
      */
-    public function allFills(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allFills(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of unique fills
         $aFills = [];
@@ -654,7 +654,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style\Font[] All fonts in PhpSpreadsheet
      */
-    public function allFonts(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allFonts(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of unique fonts
         $aFonts = [];
@@ -679,7 +679,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style\Borders[] All borders in PhpSpreadsheet
      */
-    public function allBorders(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allBorders(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of unique borders
         $aBorders = [];
@@ -704,7 +704,7 @@ class Style extends WriterPart
      *
      * @return \PhpOffice\PhpSpreadsheet\Style\NumberFormat[] All number formats in PhpSpreadsheet
      */
-    public function allNumberFormats(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function allNumberFormats(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Get an array of unique number formats
         $aNumFmts = [];
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Theme.php b/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
index 89ec67e6..4dde8153 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Theme.php
@@ -137,7 +137,7 @@ class Theme extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeTheme(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    public function writeTheme(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Create XML writer
         $objWriter = null;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
index bc1c4159..16f9f73b 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
@@ -36,7 +36,7 @@ class Workbook extends WriterPart
      *
      * @return string XML Output
      */
-    public function writeWorkbook(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null, $recalcRequired = false)
+    public function writeWorkbook(\PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet, $recalcRequired = false)
     {
         // Create XML writer
         if ($this->getParentWriter()->getUseDiskCaching()) {
@@ -128,7 +128,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeBookViews(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    private function writeBookViews(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // bookViews
         $objWriter->startElement('bookViews');
@@ -159,7 +159,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeWorkbookProtection(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    private function writeWorkbookProtection(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         if ($spreadsheet->getSecurity()->isSecurityEnabled()) {
             $objWriter->startElement('workbookProtection');
@@ -211,7 +211,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeSheets(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    private function writeSheets(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Write sheets
         $objWriter->startElement('sheets');
@@ -266,7 +266,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDefinedNames(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet = null)
+    private function writeDefinedNames(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\SpreadSheet $spreadsheet)
     {
         // Write defined names
         $objWriter->startElement('definedNames');
@@ -351,7 +351,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDefinedNameForAutofilter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null, $pSheetId = 0)
+    private function writeDefinedNameForAutofilter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet, $pSheetId = 0)
     {
         // definedName for autoFilter
         $autoFilterRange = $pSheet->getAutoFilter()->getRange();
@@ -388,7 +388,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDefinedNameForPrintTitles(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null, $pSheetId = 0)
+    private function writeDefinedNameForPrintTitles(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet, $pSheetId = 0)
     {
         // definedName for PrintTitles
         if ($pSheet->getPageSetup()->isColumnsToRepeatAtLeftSet() || $pSheet->getPageSetup()->isRowsToRepeatAtTopSet()) {
@@ -432,7 +432,7 @@ class Workbook extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDefinedNameForPrintArea(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null, $pSheetId = 0)
+    private function writeDefinedNameForPrintArea(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet, $pSheetId = 0)
     {
         // definedName for PrintArea
         if ($pSheet->getPageSetup()->isPrintAreaSet()) {
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index b1a5debd..e8d3b715 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -144,7 +144,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeSheetPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeSheetPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // sheetPr
         $objWriter->startElement('sheetPr');
@@ -192,7 +192,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDimension(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeDimension(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // dimension
         $objWriter->startElement('dimension');
@@ -208,7 +208,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeSheetViews(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeSheetViews(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // sheetViews
         $objWriter->startElement('sheetViews');
@@ -321,7 +321,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeSheetFormatPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeSheetFormatPr(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // sheetFormatPr
         $objWriter->startElement('sheetFormatPr');
@@ -374,7 +374,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeCols(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeCols(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // cols
         if (count($pSheet->getColumnDimensions()) > 0) {
@@ -440,7 +440,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeSheetProtection(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeSheetProtection(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // sheetProtection
         $objWriter->startElement('sheetProtection');
@@ -476,7 +476,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeConditionalFormatting(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeConditionalFormatting(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // Conditional id
         $id = 1;
@@ -550,7 +550,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeDataValidations(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeDataValidations(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // Datavalidation collection
         $dataValidationCollection = $pSheet->getDataValidationCollection();
@@ -617,7 +617,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeHyperlinks(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeHyperlinks(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // Hyperlink collection
         $hyperlinkCollection = $pSheet->getHyperlinkCollection();
@@ -659,7 +659,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeProtectedRanges(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeProtectedRanges(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         if (count($pSheet->getProtectedCells()) > 0) {
             // protectedRanges
@@ -689,7 +689,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeMergeCells(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeMergeCells(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         if (count($pSheet->getMergeCells()) > 0) {
             // mergeCells
@@ -715,7 +715,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writePrintOptions(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writePrintOptions(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // printOptions
         $objWriter->startElement('printOptions');
@@ -742,7 +742,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writePageMargins(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writePageMargins(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // pageMargins
         $objWriter->startElement('pageMargins');
@@ -763,7 +763,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeAutoFilter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeAutoFilter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         $autoFilterRange = $pSheet->getAutoFilter()->getRange();
         if (!empty($autoFilterRange)) {
@@ -857,7 +857,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writePageSetup(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writePageSetup(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // pageSetup
         $objWriter->startElement('pageSetup');
@@ -893,7 +893,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeHeaderFooter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeHeaderFooter(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // headerFooter
         $objWriter->startElement('headerFooter');
@@ -919,7 +919,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeBreaks(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeBreaks(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // Get row and column breaks
         $aRowBreaks = [];
@@ -1201,7 +1201,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeLegacyDrawing(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeLegacyDrawing(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // If sheet contains comments, add the relationships
         if (count($pSheet->getComments()) > 0) {
@@ -1219,7 +1219,7 @@ class Worksheet extends WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    private function writeLegacyDrawingHF(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter = null, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet = null)
+    private function writeLegacyDrawingHF(\PhpOffice\PhpSpreadsheet\Shared\XMLWriter $objWriter, \PhpOffice\PhpSpreadsheet\Worksheet $pSheet)
     {
         // If sheet contains images, add the relationships
         if (count($pSheet->getHeaderFooter()->getImages()) > 0) {
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/WriterPart.php b/src/PhpSpreadsheet/Writer/Xlsx/WriterPart.php
index 48744bb9..881683db 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/WriterPart.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/WriterPart.php
@@ -40,7 +40,7 @@ abstract class WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    public function setParentWriter(\PhpOffice\PhpSpreadsheet\Writer\IWriter $pWriter = null)
+    public function setParentWriter(\PhpOffice\PhpSpreadsheet\Writer\IWriter $pWriter)
     {
         $this->parentWriter = $pWriter;
     }
@@ -67,10 +67,8 @@ abstract class WriterPart
      *
      * @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
      */
-    public function __construct(\PhpOffice\PhpSpreadsheet\Writer\IWriter $pWriter = null)
+    public function __construct(\PhpOffice\PhpSpreadsheet\Writer\IWriter $pWriter)
     {
-        if (!is_null($pWriter)) {
-            $this->parentWriter = $pWriter;
-        }
+        $this->parentWriter = $pWriter;
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Writer/Ods/ContentTest.php b/tests/PhpSpreadsheetTests/Writer/Ods/ContentTest.php
index 941e684f..8a4aae64 100644
--- a/tests/PhpSpreadsheetTests/Writer/Ods/ContentTest.php
+++ b/tests/PhpSpreadsheetTests/Writer/Ods/ContentTest.php
@@ -38,9 +38,7 @@ class ContentTest extends \PHPUnit_Framework_TestCase
 
     public function testWriteEmptySpreadsheet()
     {
-        $content = new Content();
-        $content->setParentWriter(new Ods(new Spreadsheet()));
-
+        $content = new Content(new Ods(new Spreadsheet()));
         $xml = $content->write();
 
         $this->assertXmlStringEqualsXmlFile($this->samplesPath . '/content-empty.xml', $xml);
@@ -91,9 +89,7 @@ class ContentTest extends \PHPUnit_Framework_TestCase
         $worksheet2->setCellValue('A1', 2);
 
         // Write
-        $content = new Content();
-        $content->setParentWriter(new Ods($workbook));
-
+        $content = new Content(new Ods($workbook));
         $xml = $content->write();
 
         $this->assertXmlStringEqualsXmlFile($this->samplesPath . '/content-with-data.xml', $xml);