AttributeError: 'CHKInventoryRepository' object has no attribute 'get_ancestry'

daniel@daydream:~/bzr/ubuntu-packaging-guide$ bzr stats
getting ancestry
bzr: ERROR: exceptions.AttributeError: 'CHKInventoryRepository' object has no attribute 'get_ancestry'

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 930, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1141, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 673, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 697, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 136, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 166, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/stats/cmds.py", line 274, in run
    info = get_info(a_branch.repository, last_rev)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/stats/cmds.py", line 169, in get_info
    ancestry = a_repo.get_ancestry(revision)[1:]
AttributeError: 'CHKInventoryRepository' object has no attribute 'get_ancestry'

bzr 2.6b2 on python 2.7.3 (Linux-3.5.0-11-generic-x86_64-with-
    Ubuntu-12.10-quantal)
arguments: ['/usr/bin/bzr', 'stats']
plugins: bash_completion[2.6b2], builddeb[2.7.0dev], builder[0.7.3],
    bzrtools[2.5.0], changelog_merge[2.6b2], git[0.6.9], launchpad[2.6b2],
    netrc_credential_store[2.6b2], news_merge[2.6b2], po_merge[2.6b2],
    stats[0.2.0dev], weave_fmt[2.6b2]
encoding: 'utf-8', fsenc: 'UTF-8', lang: 'de_DE.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/bzr/+filebug
    including this traceback and a description of the problem.
daniel@daydream:~/bzr/ubuntu-packaging-guide$