[Zope] export of modified ZClass fails

[Dennis Allison]

I modified ZDiscussions significantly and it works fine. It's a ZClass
with Python base classes.   I want to export it to another Zope instance.
So I simply exported it from the Control_Panel/Products menu in the ZMI.
Export worked fine but when I import it in the new system, I get a 
diagnostic 

  Failed to import class _ZClass_for_ZDTopic from module Products.ZDBase

Products.ZDBase defines the ZDisscussions base classes.  It was installed
successfully in a separate, independent import.

Can anyone suggest what the problem might be and suggest what it is I need
to do to get things to export properly.