[Zope] Subclassing User

Hi! Can anybody tell me anything about subclassing a UserObject? I have had some neat errors (including seg-faults and indefinite loops by just subclassing a ZClass from User Object) When I tried to insantiate a Subclass of UserObject Zope quit. (I did not change anything to the default constructors and expected something like "missing argument", but not an indefinite loop :) I am using Zope 2.1.4 on Linux/x86 (2.2.13) Thanks. Regards, Ingo.