diff --git a/Classes/PHPExcel.php b/Classes/PHPExcel.php
index a78c5ce0..cc70286d 100644
--- a/Classes/PHPExcel.php
+++ b/Classes/PHPExcel.php
@@ -198,7 +198,7 @@ class PHPExcel
      * @return PHPExcel_Worksheet
      * @throws Exception
      */
-    public function createSheet($iSheetIndex = null)
+    public function createSheet($iSheetIndex = NULL)
     {
         $newSheet = new PHPExcel_Worksheet($this);
         $this->addSheet($newSheet, $iSheetIndex);
@@ -224,13 +224,16 @@ class PHPExcel
      * @return PHPExcel_Worksheet
      * @throws Exception
      */
-    public function addSheet(PHPExcel_Worksheet $pSheet, $iSheetIndex = null)
+    public function addSheet(PHPExcel_Worksheet $pSheet, $iSheetIndex = NULL)
     {
 		if ($this->sheetNameExists($pSheet->getTitle())) {
 			throw new Exception("Workbook already contains a worksheet named '{$pSheet->getTitle()}'. Rename this worksheet first.");
 		}
 
         if($iSheetIndex === NULL) {
+            if ($this->_activeSheetIndex < 0) {
+            	$this->_activeSheetIndex = 0;
+            }
             $this->_workSheetCollection[] = $pSheet;
         } else {
             // Insert the sheet at the requested index
diff --git a/changelog.txt b/changelog.txt
index 193fba8b..2d4d439b 100644
--- a/changelog.txt
+++ b/changelog.txt
@@ -108,6 +108,7 @@ Fixed in develop branch:
 - Bugfix:   (wolf5x)     Work item 18492 - load file failed if the file has no extensionType
 - Bugfix:   (dverspui)   Pattern fill colours in Excel2007 Style Writer
 - Bugfix:   (MBaker)     Excel2007 Writer order of font style elements to conform with Excel2003 using compatibility pack
+- Bugfix:   (MBaker)     Work item 18425 - Problems with $_activeSheetIndex when decreased below 0.
 
 
 2012-05-19 (v1.7.7):