Hi mariano I have the impression that methodDict should be redefined as you propose it because else it does not work well with the cannotInterpret hook. I suggest to redefine methodDict as follow. notice the cool comment!!! If this is ok -> open a bug entry. Behavior >> methodDict "The method dictionary of a class can be nil when we want to use the cannotInterpret: hook. Indeed when a class dictionary is nil, the VM sends the message cannotInterpret: to the receiver but starting the look up in the superclass. Now the system relies that when the message methodDict is sent to a class a method dictionary is returned. The implementation below makes sure that a method dictionary can be nilled but does not confuse the system." methodDict == nil ifTrue: [^ MethodDictionary new ]. ^ methodDict Stef