missing this_branch during 'bzr switch'

  affects bzr

When running 'bzr switch' I get the attached exception. (Not setting
importance since I don't know how serious this is yet).

BzrVersion: 2.1.1
CommandLine: ['/usr/bin/bzr', 'switch', 'oops-on-pool-overwrite-error']

 Traceback (most recent call last):
   File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 853, in exception_to_return_code
     return the_callable(*args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 1055, in run_bzr
     ret = run(*run_argv)
   File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 661, in run_argv_aliases
     return self.run_direct(**all_cmd_args)
   File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 665, in run_direct
     return self._operation.run_simple(*args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 122, in run_simple
     self.cleanups, self.func, *args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 156, in _do_with_cleanups
     result = func(*args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/builtins.py", line 5519, in run
     switch.switch(control_dir, to_branch, force, revision_id=revision)
   File "/usr/lib/python2.6/dist-packages/bzrlib/switch.py", line 40, in switch
     _update(tree, source_repository, quiet, revision_id)
   File "/usr/lib/python2.6/dist-packages/bzrlib/switch.py", line 138, in _update
     merge.Merge3Merger(tree, tree, base_tree, to_branch.repository.revision_tree(revision_id))
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 755, in __init__
     self.do_merge()
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 766, in do_merge
     operation.run()
   File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 118, in run
     self.cleanups, self.func, self, *args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 156, in _do_with_cleanups
     result = func(*args, **kwargs)
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 772, in _do_merge
     self._compute_transform()
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 814, in _compute_transform
     file_status = self._do_merge_contents(file_id)
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 1302, in _do_merge_contents
     hook_status, lines = hook.merge_contents(params)
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 161, in merge_contents
     not self.filename_matches_config(params)):
   File "/usr/lib/python2.6/dist-packages/bzrlib/merge.py", line 133, in filename_matches_config
     config = self.merger.this_branch.get_config()
 AttributeError: 'NoneType' object has no attribute 'get_config'