bzr log --short broken over hpss

Observe:

mwh@grond:~$ bzr log --short bzr+ssh://localhost/home/mwh/canonical/repos/loggerhead/trunk
bzr: ERROR: bzrlib.errors.ObjectNotLocked: _KnitGraphIndex(CombinedGraphIndex()) is not locked

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 857, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 797, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 499, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 818, in ignore_pipe
    result = func(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 1709, in run
    limit=limit)
  File "/usr/lib/python2.5/site-packages/bzrlib/log.py", line 185, in show_log
    start_revision, end_revision, search, limit)
  File "/usr/lib/python2.5/site-packages/bzrlib/log.py", line 232, in _show_log
    branch.repository, view_revisions, generate_delta):
  File "/usr/lib/python2.5/site-packages/bzrlib/log.py", line 305, in _iter_revisions
    cur_view_revisions = [d for x, d in zip(range(num), view_revisions)]
  File "/usr/lib/python2.5/site-packages/bzrlib/log.py", line 297, in _linear_view_revisions
    for num, revision_id in enumerate(revision_ids):
  File "/usr/lib/python2.5/site-packages/bzrlib/repository.py", line 1595, in iter_reverse_revision_history
    parents = graph.get_parent_map([next_id])[next_id]
  File "/usr/lib/python2.5/site-packages/bzrlib/graph.py", line 132, in get_parent_map
    new_parents = self._real_provider.get_parent_map(needed)
  File "/usr/lib/python2.5/site-packages/bzrlib/repository.py", line 1746, in get_parent_map
    self.revisions.get_parent_map(query_keys).iteritems():
  File "/usr/lib/python2.5/site-packages/bzrlib/knit.py", line 1069, in get_parent_map
    return self._get_parent_map_with_sources(keys)[0]
  File "/usr/lib/python2.5/site-packages/bzrlib/knit.py", line 1088, in _get_parent_map_with_sources
    new_result = source.get_parent_map(missing)
  File "/usr/lib/python2.5/site-packages/bzrlib/knit.py", line 2232, in get_parent_map
    self._check_read()
  File "/usr/lib/python2.5/site-packages/bzrlib/knit.py", line 2115, in _check_read
    raise errors.ObjectNotLocked(self)
ObjectNotLocked: _KnitGraphIndex(CombinedGraphIndex()) is not locked

bzr 1.6b3 on python 2.5.2 (linux2)
arguments: ['/usr/bin/bzr', 'log', '--short', 'bzr+ssh://localhost/home/mwh/canonical/repos/loggerhead/trunk']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_NZ.UTF-8'
plugins:
  bpm /home/mwh/.bazaar/plugins/bpm [unknown]
  bzrtools /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [1.5.0]
  gtk /home/mwh/.bazaar/plugins/gtk [0.95.0dev1]
  launchpad /usr/lib/python2.5/site-packages/bzrlib/plugins/launchpad [unknown]
  loggerserve /home/mwh/.bazaar/plugins/loggerserve [unknown]
  loom /home/mwh/.bazaar/plugins/loom [1.4.0dev0]
  lpreview /home/mwh/.bazaar/plugins/lpreview [unknown]
  merged /home/mwh/.bazaar/plugins/merged [unknown]
  pqm /home/mwh/.bazaar/plugins/pqm [1.4.0dev0]
  pybloom /home/mwh/.bazaar/plugins/pybloom [unknown]
  search /home/mwh/.bazaar/plugins/search [1.6.0dev0]
  service /home/mwh/.bazaar/plugins/service [unknown]
  svn /home/mwh/.bazaar/plugins/svn [0.4.11dev0]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.

I noticed by chance that this doesn't seem to affect bzr log --short of a lightweight checkout of a branch, which seems a little odd. It also affects log --line but not log --long

It happens over bzr:// too.