diff --git a/pyglance/glance/data.py b/pyglance/glance/data.py
index 3c36ef58611466cbed312dfabb58a34a7f01cbfa..417a7ba15e7749910920e833aa300e932a827431 100644
--- a/pyglance/glance/data.py
+++ b/pyglance/glance/data.py
@@ -332,6 +332,9 @@ class DiffInfoObject (object) :
                                                                                      aDataObject.select_fill_value(),
                                                                                      bDataObject.select_fill_value())
         
+        # we can't continue if we don't have a fill value
+        assert(fill_data_value is not None)
+        
         # construct our diff'ed data set
         raw_diff = np.zeros(shape, dtype=sharedType)
         raw_diff[~valid_in_both] = fill_data_value # throw away invalid data