--- a/web/views/undohistory.py	Mon Feb 27 11:51:09 2012 +0100
+++ b/web/views/undohistory.py	Mon Feb 27 11:53:24 2012 +0100
@@ -41,7 +41,7 @@
     # XXX FIXME : this selector should be completed to allow selection on the
     # entity or relation types and public / private.
     def __init__(self, action_type='CUDAR'):
-        assert not set(action_type) & set('CUDAR')
+        assert not set(action_type) - set('CUDAR')
         self.action_type = action_type
 
     def __str__(self):