diff -r 53fbd5644bff -r c655e19cbc35 cubicweb/server/migractions.py
--- a/cubicweb/server/migractions.py	Wed May 25 09:51:51 2016 +0200
+++ b/cubicweb/server/migractions.py	Tue Jun 07 10:05:30 2016 +0200
@@ -1369,7 +1369,7 @@
             getattr(entity, attribute)
             storage.migrate_entity(entity, attribute)
             # remove from entity cache to avoid memory exhaustion
-            del entity.cw_attr_cache[attribute]
+            entity.cw_attr_cache.pop(attribute, None)
             pb.update()
         print()
         source.set_storage(etype, attribute, storage)