bzr: ERROR: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process /usr/bin/bzr exited with status 4

bminton@bminton:~/src$ bzr branch lp:calibre
bzr: ERROR: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process /usr/bin/bzr exited with status 4

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/builtins.py", line 1508, in run
    source_branch=br_from)
  File "/usr/lib/python2.7/dist-packages/bzrlib/bzrdir.py", line 366, in sprout
    create_tree_if_local=create_tree_if_local)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 132, in run
    self.cleanups, self.func, self, *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/bzrdir.py", line 426, in _sprout
    repository=result_repo)
  File "/usr/lib/python2.7/dist-packages/bzrlib/decorators.py", line 155, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 1290, in sprout
    self.copy_content_into(result, revision_id=revision_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 1332, in copy_content_into
    revision_id=revision_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/decorators.py", line 218, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 3309, in copy_content_into
    self.source._synchronize_history(self.target, revision_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 1323, in _synchronize_history
    destination.set_last_revision_info(revno, revision_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/decorators.py", line 218, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 2538, in set_last_revision_info
    self._run_post_change_branch_tip_hooks(old_revno, old_revid)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 1215, in _run_post_change_branch_tip_hooks
    hook(params)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/dbus/hook.py", line 39, in on_post_change_branch_tip
    activity.advertise_branch(params.branch)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/dbus/activity.py", line 78, in advertise_branch
    self.announce_revision(branch.last_revision(), branch.base)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/dbus/activity.py", line 89, in announce_revision
    self._call_on_broadcast('announce_revision', revision, url)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/dbus/activity.py", line 109, in _call_on_broadcast
    Broadcast.DBUS_PATH)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 241, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 248, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 180, in activate_name_owner
    self.start_service_by_name(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 278, in start_service_by_name
    'su', (bus_name, flags)))
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 651, in call_blocking
    message, timeout)
DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process /usr/bin/bzr exited with status 4

bzr 2.6.0dev2 on python 2.7.3 (Linux-3.7.0-x86_64-with-debian-7.0)
arguments: ['/usr/bin/bzr', 'branch', 'lp:calibre']
plugins: bash_completion[2.6.0dev2], bzrtools[2.5.0],
    changelog_merge[2.6.0dev2], dbus[0.1.0dev], etckeeper[unknown],
    gtk[0.104.0dev], launchpad[2.6.0dev2], loom[2.2.0],
    netrc_credential_store[2.6.0dev2], news_merge[2.6.0dev2],
    po_merge[2.6.0dev2], rewrite[0.6.4dev], search[1.7.0dev], svn[1.2.1],
    weave_fmt[2.6.0dev2]
encoding: 'ascii', fsenc: 'utf8', lang: None

*** 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.