running 'brz ignore' in a git tree breaks

[Jelmer Vernooij]

blodeuwedd:~/src/dulwich% bzr ignore foo
brz: ERROR: TypeError: join() argument must be str or bytes, not 'NoneType'

Traceback (most recent call last):
  File "/home/jelmer/src/breezy/breezy/commands.py", line 1016, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/jelmer/src/breezy/breezy/commands.py", line 1202, in run_bzr
    ret = run(*run_argv)
  File "/home/jelmer/src/breezy/breezy/commands.py", line 759, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/jelmer/src/breezy/breezy/commands.py", line 784, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/jelmer/src/breezy/breezy/cleanup.py", line 137, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/jelmer/src/breezy/breezy/cleanup.py", line 166, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/jelmer/src/breezy/breezy/builtins.py", line 3233, in run
    ignores.tree_ignores_add_patterns(tree, name_pattern_list)
  File "/home/jelmer/src/breezy/breezy/ignores.py", line 190, in tree_ignores_add_patterns
    ifn = tree.abspath(tree._format.ignore_filename)
  File "/home/jelmer/src/breezy/breezy/workingtree.py", line 318, in abspath
    return osutils.pathjoin(self.basedir, filename)
  File "/usr/lib/python3.6/posixpath.py", line 94, in join
    genericpath._check_arg_types('join', a, *p)
  File "/usr/lib/python3.6/genericpath.py", line 149, in _check_arg_types
    (funcname, s.__class__.__name__)) from None
TypeError: join() argument must be str or bytes, not 'NoneType'

brz 3.0a2 on python 3.6.7 (Linux-4.18.0-2-amd64-x86_64-with-debian-buster-sid)
arguments: ['/home/jelmer/src/breezy/brz', '-Dhpss', 'ignore', 'foo']
plugins: apt[unknown], bash_completion[3.0a2], brztools[unknown],
    changelog_merge[3.0a2], commitfromnews[unknown], cvs[3.0a2],
    cvsps_import[unknown], darcs[unknown], debian[2.8.6], email[unknown],
    fastimport[unknown], grep[3.0a2], jelmer[unknown], launchpad[3.0a2],
    loggerhead[1.20.0], mtn[3.0a2], netrc_credential_store[3.0a2],
    news_merge[3.0a2], po_merge[3.0a2], propose[unknown], qbrz[0.23.2],
    repodebug[unknown], stats[3.0a2], upload[3.0a2], weave_fmt[3.0a2]
encoding: 'utf-8', fsenc: 'utf-8', lang: 'en_GB.UTF-8'

*** Bazaar has encountered an internal error. This probably indicates a
    bug in Bazaar. You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/brz/+filebug
    including this traceback and a description of the problem.
brz: warning: some compiled extensions could not be loaded; see ``brz help missing-extensions``