diff --git a/src/PhpSpreadsheet/Shared/StringHelper.php b/src/PhpSpreadsheet/Shared/StringHelper.php
index d1422c76c7..435d0e4cda 100644
--- a/src/PhpSpreadsheet/Shared/StringHelper.php
+++ b/src/PhpSpreadsheet/Shared/StringHelper.php
@@ -490,7 +490,7 @@ public static function substring($textValue, $offset, $length = 0)
      */
     public static function strToUpper($textValue)
     {
-        return mb_convert_case($textValue, MB_CASE_UPPER, 'UTF-8');
+        return mb_convert_case($textValue ?? '', MB_CASE_UPPER, 'UTF-8');
     }
 
     /**