[pipeline] bzr switch PIPENAME crashes with bzr: ERROR: exceptions.AttributeError: 'module' object has no attribute 'BzrBranch5'

bzr: ERROR: exceptions.AttributeError: 'module' object has no attribute 'BzrBranch5'

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/plugins/loom/commands.py", line 259, in run_argv_aliases
    super(cmd_switch, self).run_argv_aliases(list(argv), alias_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/loom/commands.py", line 239, in run
    tree = LoomTreeDecorator(tree)
  File "/usr/lib/python2.7/dist-packages/bzrlib/lazy_import.py", line 116, in __call__
    obj = object.__getattribute__(self, '_resolve')()
  File "/usr/lib/python2.7/dist-packages/bzrlib/lazy_import.py", line 85, in _resolve
    obj = factory(self, scope, name)
  File "/usr/lib/python2.7/dist-packages/bzrlib/lazy_import.py", line 200, in _import
    module = __import__(module_python_path, scope, scope, [member], level=0)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/loom/tree.py", line 36, in <module>
    from branch import EMPTY_REVISION
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/loom/branch.py", line 747, in <module>
    class LoomBranch(LoomSupport, bzrlib.branch.BzrBranch5):
AttributeError: 'module' object has no attribute 'BzrBranch5'

bzr 2.6b2 on python 2.7.3 (Linux-3.5.0-4-generic-x86_64-with-
    Ubuntu-12.10-quantal)
arguments: ['/usr/bin/bzr', 'switch', ':first']
plugins: bash_completion[2.6b2], builddeb[2.7.0dev], builder[0.7.3],
    bzrtools[2.5.0], changelog_merge[2.6b2], cvsps_import[unknown],
    dbus[0.1.0dev], email[unknown], explorer[1.3.0dev], fastimport[0.13.0],
    git[0.6.9], grep[0.5.0dev], gtk[0.104.0dev], launchpad[2.6b2],
    loom[2.2.0], netrc_credential_store[2.6b2], news_merge[2.6b2],
    pipeline[1.4.0], po_merge[2.6b2], pqm[1.4.0dev], qbzr[0.22.2],
    rewrite[0.6.4dev], search[1.7.0dev], stats[0.2.0dev], svn[1.2.1],
    trac[1.0.0dev], upload[1.1.0], weave_fmt[2.6b2], webdav[1.12.2],
    xmloutput[0.8.8]
encoding: 'utf-8', fsenc: 'UTF-8', lang: 'en_AU.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

$ apt-cache policy bzr
bzr:
  Installed: 2.6.0~beta2-0ubuntu1
  Candidate: 2.6.0~beta2-0ubuntu1
  Version table:
 *** 2.6.0~beta2-0ubuntu1 0
        500 http://au.archive.ubuntu.com/ubuntu/ quantal/main amd64 Packages
        100 /var/lib/dpkg/status

$ bzr --version
Bazaar (bzr) 2.6b2
  Python interpreter: /usr/bin/python 2.7.3
  Python standard library: /usr/lib/python2.7
  Platform: Linux-3.5.0-4-generic-x86_64-with-Ubuntu-12.10-quantal
  bzrlib: /usr/lib/python2.7/dist-packages/bzrlib
  Bazaar configuration: /home/smspillaz/.bazaar
  Bazaar log file: /home/smspillaz/.bzr.log

Copyright 2005-2012 Canonical Ltd.
http://bazaar.canonical.com/

bzr comes with ABSOLUTELY NO WARRANTY. bzr is free software, and
you may use, modify and redistribute it under the terms of the GNU
General Public License version 2 or later.

Bazaar is part of the GNU Project to produce a free operating system.