Comment 1 for bug 248932

I've got a backtrace for you:

bzrlib/commands.py(848)main()
-> ret = run_bzr_catch_errors(argv)
bzrlib/commands.py(857)run_bzr_catch_errors()
-> return run_bzr(argv)
bzrlib/commands.py(797)run_bzr()
-> ret = run(*run_argv)
bzrlib/commands.py(499)run_argv_aliases()
-> return self.run(**all_cmd_args)
bzrlib/builtins.py(4417)run()
-> reconfiguration.apply(force)
bzrlib/reconfigure.py(258)apply()
-> self.local_branch.last_revision())
bzrlib/repository.py(963)fetch()
-> raise errors.IncompatibleRepositories(source, self)

I did some more debugging. The immediate cause seems to be a NotImplementedError raised by InterRepository.fetch, which is due to the lack of a specialized InterRepository provider, which in turn is due to the fact that the _same_model check fails because source.supports_rich_root() == True but target.supports_rich_root() == False.