diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index b6a6fc39..8faa7ae2 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -1079,7 +1079,7 @@ class Worksheet extends WriterPart
     {
         $objWriter->writeAttribute('t', $mappedType);
         if (!$cellValue instanceof RichText) {
-            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue]);
+            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue] ?? '');
         } else {
             $objWriter->writeElement('v', $pFlippedStringTable[$cellValue->getHashCode()]);
         }