[Zope] Handling of objects returned from external methods within zope?

Hi, I'm using Zope 2.5.0 on Linux. I have created an external method in Zope which catalogs and external function. This function just creates on object of a class defined in the external file and returns this. The class looks like this: class x: def y (s): return 1 Within Zope I now try to call the method y() of the object that is returned by the external function. This does not work - zope want's me to (re)login immediately. I guess I'm just not allowed to use classes defined outside of zope within zope. Is there some way to register a class so that it's objects can be used inside of Zope? If so, how can I achieve this? Thanks, Stephan