"Could not understand response from smart server" when diffing lightweight checkout of smart server branch

In an existing lightweight checkout of a branch from Launchpad with a local change, running diff generates an error:

$ bzr -Dhpss -Dhpssdetail --no-plugins diff
=== modified file 'sourcedeps.conf'
bzr: ERROR: Could not understand response from smart server: ['x\x9c\xa5\x92\xcb\x0e\x820\x10E\xd7\xf2\x15\xee\r\xad\xa0\x10\x1f\xf1cJ[\xa1\x81>\x9c\xb6$\xb2\xf0\xdb\x85\x80Q0\x01\x13\x17\x93Lf\xe6\xde\x93\xde\x14a!\x8d\x06\x17Z']
HPSS calls: 7 (0 vfs) SmartSSHClientMedium(bzr+ssh://<email address hidden>/)

From ~/.bzr.log:

Wed 2012-09-05 12:16:12 +0100
0.040 bazaar version: 2.6.0dev3
0.040 bzr arguments: [u'-Dhpss', u'-Dhpssdetail', u'--no-plugins', u'diff']
0.045 encoding stdout as sys.stdout encoding 'UTF-8'
0.071 hpss: Built a new medium: SmartSSHClientMedium
0.072 hpss call: 'BzrDir.open_2.1', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/'
0.072 (to bzr+ssh://bazaar.launchpad.net/~canonical-ca-hackers/ca-configs/pkgimport-configmanager/)
0.095 ssh implementation is OpenSSH
3.137 result: ('yes', 'no')
3.138 hpss call: 'BzrDir.open_branchV3', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/'
3.138 (to bzr+ssh://bazaar.launchpad.net/~canonical-ca-hackers/ca-configs/pkgimport-configmanager/)
3.239 result: ('branch', 'Bazaar Branch Format 7 (needs bzr 1.6)\n')
3.240 hpss call: 'BzrDir.find_repositoryV3', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/'
3.240 (to bzr+ssh://bazaar.launchpad.net/~canonical-ca-hackers/ca-configs/pkgimport-configmanager/)
3.341 result: ('ok', '', 'yes', 'no', 'yes', 'Bazaar repository format 2a (needs bzr 1.16 or later)\n')
3.346 hpss call: 'Branch.get_stacked_on_url', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/'
3.346 (to bzr+ssh://bazaar.launchpad.net/~canonical-ca-hackers/ca-configs/pkgimport-configmanager/)
3.445 result: ('NotStacked',)
3.446 opening working tree '/home/jml/src/ca-configs/pkgimport-configmanager'
3.453 hpss call: 'Branch.get_config_file', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/'
3.453 (to bzr+ssh://bazaar.launchpad.net/~canonical-ca-hackers/ca-configs/pkgimport-configmanager/)
3.548 result: ('ok',)
3.548 0 body bytes read
3.555 hpss call w/body: 'Repository.iter_revisions', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/' ('james.westby@canonic'...)
3.555 58 bytes
3.651 result: ('ok', '10')
3.652 291 byte part read
3.655 hpss call w/body: 'Repository.iter_files_bytes', '~canonical-ca-hackers/ca-configs/pkgimport-configmanager/' ('sourcedeps.conf-2012'...)
3.655 108 bytes
3.751 result: ('ok',)
3.751 5 byte part read
3.752 2 byte part read
3.752 198 byte part read
3.752 Transferred: 2kB (0.7kB/s r:1kB w:1kB)
3.767 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/commands.py", line 1158, in ignore_pipe
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/builtins.py", line 2383, in run
    format_cls=format)
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 460, in show_diff_trees
    return differ.show_diff(specific_files, extra_trees)
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 956, in show_diff
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 1016, in _show_diff
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 1042, in _diff
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 522, in _diff_many
    new_kind)
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 662, in diff
    old_path, new_path)
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 682, in diff_text
    from_text = _get_text(self.old_tree, from_file_id, from_path)
  File "/usr/lib/python2.7/dist-packages/bzrlib/diff.py", line 678, in _get_text
    return tree.get_file_lines(file_id, path)
  File "/usr/lib/python2.7/dist-packages/bzrlib/tree.py", line 322, in get_file_lines
    return osutils.split_lines(self.get_file_text(file_id, path))
  File "/usr/lib/python2.7/dist-packages/bzrlib/workingtree_4.py", line 1904, in get_file_text
    _, content = list(self.iter_files_bytes([(file_id, None)]))[0]
  File "/usr/lib/python2.7/dist-packages/bzrlib/remote.py", line 2224, in iter_files_bytes
    desired_files, absent):
  File "/usr/lib/python2.7/dist-packages/bzrlib/remote.py", line 2212, in _iter_files_bytes_rpc
    raise errors.UnexpectedSmartServerResponse(args)
UnexpectedSmartServerResponse: Could not understand response from smart server: ['x\x9c\xa5\x92\xcb\x0e\x820\x10E\xd7\xf2\x15\xee\r\xad\xa0\x10\x1f\xf1cJ[\xa1\x81>\x9c\xb6$\xb2\xf0\xdb\x85\x80Q0\x01\x13\x17\x93Lf\xe6\xde\x93\xde\x14a!\x8d\x06\x17Z']

3.767 return code 3
[22797] 2012-09-05 12:16:16.161 INFO: HPSS calls: 7 (0 vfs) SmartSSHClientMedium(bzr+ssh://<email address hidden>/)