Which version of python are you running?  I can't match that traceback up to the code in the current 2.7 inspect module.

That said, the same issue probably exists in the current code.  A 2.7 class is expected to have either an __mro__ or a __bases__ attribute, and if it has neither inspect probably *should* throw an error, since it can't know what to do with the class.  The error could be clearer, though.

But, let's see what others think.