bzr merge --uncommitted of self results in traceback

Obviously this doesn't make much sense, but it would be nice if we could give a better error message:

ganieda:~/bzr/foreign-features% bzr merge --uncommitted .
bzr: ERROR: exceptions.AssertionError: file already locked: u'/home/jelmer/bzr/foreign-features/.bzr/checkout/dirstate'

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 729, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 924, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 560, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/jelmer/.bazaar/dev-plugins/qbzr/lib/commands.py", line 664, in run
    return bzrlib.builtins.cmd_merge.run(self, *args, **kw)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 3591, in run
    verified)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 3609, in _do_merge
    conflict_count = merger.do_merge()
  File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 498, in do_merge
    self.other_tree.unlock()
  File "/usr/lib/python2.5/site-packages/bzrlib/workingtree_4.py", line 1166, in unlock
    self._dirstate.save()
  File "/usr/lib/python2.5/site-packages/bzrlib/dirstate.py", line 2150, in save
    grabbed_write_lock, new_lock = self._lock_token.temporary_write_lock()
  File "/usr/lib/python2.5/site-packages/bzrlib/lock.py", line 239, in temporary_write_lock
    % (self.filename,))
AssertionError: file already locked: u'/home/jelmer/bzr/foreign-features/.bzr/checkout/dirstate'

bzr 1.15 on python 2.5.4 (linux2)
arguments: ['/usr/bin/bzr', 'merge', '--uncommitted', '.']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'
plugins:
  avahi /usr/lib/python2.5/site-packages/bzrlib/plugins/avahi [0.3dev]
  benchserializer /home/jelmer/.bazaar/plugins/benchserializer.py [unknown]
  builddeb /home/jelmer/.bazaar/plugins/builddeb [unknown]
  builder /home/jelmer/.bazaar/plugins/builder [unknown]
  bzrtools /home/jelmer/.bazaar/plugins/bzrtools [1.15]
  cia /home/jelmer/.bazaar/plugins/cia [1.0dev]
  code_swarm /home/jelmer/.bazaar/plugins/code_swarm.py [unknown]
  compressbench /home/jelmer/.bazaar/plugins/compressbench [unknown]
  cvsps_import /usr/lib/python2.5/site-packages/bzrlib/plugins/cvsps_import [unknown]
  dbus /usr/lib/python2.5/site-packages/bzrlib/plugins/dbus [0.1dev]
  devtools /home/jelmer/.bazaar/plugins/devtools [unknown]
  email /home/jelmer/.bazaar/plugins/email [unknown]
  etckeeper /usr/lib/python2.5/site-packages/bzrlib/plugins/etckeeper [unknown]
  extmerge /home/jelmer/.bazaar/plugins/extmerge [unknown]
  fastimport /usr/lib/python2.5/site-packages/bzrlib/plugins/fastimport [0.8dev]
  git /home/jelmer/.bazaar/plugins/git [0.3.3dev]
  global_log /home/jelmer/.bazaar/plugins/global_log [unknown]
  gtk /home/jelmer/.bazaar/plugins/gtk [0.96.0.dev.1]
  hg /home/jelmer/.bazaar/plugins/hg [unknown]
  jelmer /home/jelmer/.bazaar/plugins/jelmer [unknown]
  launchpad /usr/lib/python2.5/site-packages/bzrlib/plugins/launchpad [1.15]
  local_test_server /home/jelmer/.bazaar/plugins/local_test_server [unknown]
  loom /home/jelmer/.bazaar/plugins/loom.py [unknown]
  netrc_credential_store /usr/lib/python2.5/site-packages/bzrlib/plugins/netrc_credential_store [1.15]
  nm /home/jelmer/.bazaar/plugins/nm [unknown]
  pqm /usr/lib/python2.5/site-packages/bzrlib/plugins/pqm [1.4dev]
  qbzr /home/jelmer/.bazaar/plugins/qbzr [0.11dev]
  rebase /usr/lib/python2.5/site-packages/bzrlib/plugins/rebase [0.5]
  removable /home/jelmer/.bazaar/plugins/removable [unknown]
  remove_revisions /home/jelmer/.bazaar/plugins/remove_revisions [unknown]
  search /home/jelmer/.bazaar/plugins/search [1.7dev]
  stats /usr/lib/python2.5/site-packages/bzrlib/plugins/stats [unknown]
  svn /usr/lib/python2.5/site-packages/bzrlib/plugins/svn [0.6.2dev]
  upload /home/jelmer/.bazaar/plugins/upload [1.0dev]
  webdav /usr/lib/python2.5/site-packages/bzrlib/plugins/webdav [1.12]
  xmloutput /home/jelmer/.bazaar/plugins/xmloutput [0.8.3]
*** 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.