unable to build credits (bzr 2.6.0)

With a repository checkout of bzr-gtk I am unable to build because "build_credits" (credits.pickle) fails (for bzr 2.6.0)

Steps to reproduce:

$ bzr branch lp:bzr-gtk
Branched 796 revisions.

$ cd bzr-gtk/
$ python2 setup.py build_credits
running build_credits
Traceback (most recent call last):
  File "setup.py", line 209, in <module>
    'check': Check}
  File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
    dist.run_commands()
  File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "setup.py", line 89, in run
    credits = find_credits(branch.repository, branch.last_revision())
  File "/usr/lib/python2.7/site-packages/bzrlib/plugins/stats/cmds.py", line 375, in find_credits
    revs = repository.get_revisions(ancestry)
  File "<string>", line 4, in get_revisions_read_locked
  File "/usr/lib/python2.7/site-packages/bzrlib/vf_repository.py", line 1398, in get_revisions
    return self._get_revisions(revision_ids)
  File "<string>", line 4, in _get_revisions_read_locked
  File "/usr/lib/python2.7/site-packages/bzrlib/vf_repository.py", line 1406, in _get_revisions
    raise errors.NoSuchRevision(self, revid)
bzrlib.errors.NoSuchRevision: CHKInventoryRepository('file:///home/jonnyjd/bzr/bzr-gtk/.bzr/repository/') has no revision null: