diff --git a/cardstock/propertyInspector.py b/cardstock/propertyInspector.py
index 57a1404..8ae4aa1 100644
--- a/cardstock/propertyInspector.py
+++ b/cardstock/propertyInspector.py
@@ -188,7 +188,8 @@ def SetValueForKey(self, key, valType):
                 self.SetCellValue(row, 1, val)
 
     def SetValue(self, key, val):
-        self.data[key] = val
+        if key in self.data:
+            self.data[key] = val
 
     def UpdateValues(self):
         """ Update the values for the existing set of data.  (Assumes keys and types have not changed.) """