diff -r fe53c7e1625f -r 8c29253105ba server/migractions.py
--- a/server/migractions.py	Mon Feb 13 15:15:24 2012 +0100
+++ b/server/migractions.py	Tue Feb 14 15:14:40 2012 +0100
@@ -1349,7 +1349,7 @@
             getattr(entity, attribute)
             storage.migrate_entity(entity, attribute)
             # remove from entity cache to avoid memory exhaustion
-            del entity[attribute]
+            del entity.cw_attr_cache[attribute]
             pb.update()
         print
         source.set_storage(etype, attribute, storage)