diff --git a/PROMS/ReferencedObjects/Exe/RefObj/ROEditor/RODefFrm.cs b/PROMS/ReferencedObjects/Exe/RefObj/ROEditor/RODefFrm.cs
index f2dc6f34..e60a329c 100644
--- a/PROMS/ReferencedObjects/Exe/RefObj/ROEditor/RODefFrm.cs
+++ b/PROMS/ReferencedObjects/Exe/RefObj/ROEditor/RODefFrm.cs
@@ -1584,7 +1584,7 @@ namespace ROEditor
 					if (i + 1 < InUseList.Count) inuserecs = inuserecs + " ";
 				}
 			}
-			if (inuserecs != origFieldsInUse)
+			if ((inuserecs ?? "") != (origFieldsInUse ?? ""))
 				return true;
 
 
@@ -1599,7 +1599,7 @@ namespace ROEditor
 					if (i + 1 < InUseApplcList.Count) applicfieldrecs = applicfieldrecs + " ";
 				}
 			}
-			if (applicfieldrecs != origApplicFields)
+			if ((applicfieldrecs ?? "") != (origApplicFields ?? ""))
 				return true;
 
 			//nothing has changed