Index: django/db/models/query.py
===================================================================
--- django/db/models/query.py	(revision 5128)
+++ django/db/models/query.py	(working copy)
@@ -827,6 +827,9 @@
             # all uses of None as a query value.
             if lookup_type != 'exact':
                 raise ValueError, "Cannot use None as a query value"
+            else:
+                lookup_type = 'isnull'
+                value = True
         elif callable(value):
             value = value()