diff --git a/OpenXmlFormats/Spreadsheet/Sheet/CT_Cell.cs b/OpenXmlFormats/Spreadsheet/Sheet/CT_Cell.cs
index 560042ab2..6eed7ac92 100644
--- a/OpenXmlFormats/Spreadsheet/Sheet/CT_Cell.cs
+++ b/OpenXmlFormats/Spreadsheet/Sheet/CT_Cell.cs
@@ -130,7 +130,7 @@ public bool IsSetT()
         }
         public bool IsSetS()
         {
-            return sField != 0;
+            return sField != null && sField != 0;
         }
         public bool IsSetF()
         {