[Zope] export of modified ZClass fails

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.