error upgrading emacs to gc-plain-chk255

Trying to upgrade emacs-merges from 1.9 format to gc-plain-chk255 fails. This is using brisbane-core rev 3845 (with John's tweak to xml8.py) and groupcompress rev 56. Error is ...

ian@humpback:~/scm-benchmark/scratch/emacs-chk$ time bzr-brisbane upgrade --gc-plain-chk255
starting upgrade of file:///home/ian/scm-benchmark/scratch/emacs-chk/
making backup of file:///home/ian/scm-benchmark/scratch/emacs-chk/.bzr
  to file:///home/ian/scm-benchmark/scratch/emacs-chk/backup.bzr
starting repository conversion
bzr: ERROR: exceptions.TypeError: 'NoneType' object is unsubscriptable

Traceback (most recent call last):
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/commands.py", line 921, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/commands.py", line 866, in run_bzr
    ret = run(*run_argv)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/commands.py", line 547, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/builtins.py", line 2893, in run
    upgrade(url, format)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/upgrade.py", line 75, in upgrade
    Convert(url, format)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/upgrade.py", line 42, in __init__
    self.convert()
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/upgrade.py", line 69, in convert
    self.bzrdir = converter.convert(self.bzrdir, self.pb)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/bzrdir.py", line 2657, in convert
    converter.convert(repo, pb)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 3661, in convert
    self.source_repo.copy_content_into(converted)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 1101, in copy_content_into
    return InterRepository.get(self, destination).copy_content(revision_id)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 2845, in copy_content
    self.target.fetch(self.source, revision_id=revision_id)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 1161, in fetch
    find_ghosts=find_ghosts)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 3459, in fetch
    self._fetch_all_revisions(revision_ids, pb)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 3429, in _fetch_all_revisions
    basis_id = self._fetch_batch(batch, basis_id, cache)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/repository.py", line 3394, in _fetch_batch
    not self.target._format._fetch_uses_deltas))
  File "/home/ian/.bazaar/plugins/groupcompress/groupcompress.py", line 669, in insert_record_stream
    for _ in self._insert_record_stream(stream):
  File "/home/ian/.bazaar/plugins/groupcompress/groupcompress.py", line 717, in _insert_record_stream
    bytes = adapter.get_bytes(record)
  File "/home/ian/bzr/repo/brisbane-core/bzrlib/knit.py", line 229, in get_bytes
    [compression_parent], 'unordered', True).next()
  File "/home/ian/.bazaar/plugins/groupcompress/groupcompress.py", line 625, in get_record_stream
    present_keys = sorted(locations.iterkeys(), key=get_group)
  File "/home/ian/.bazaar/plugins/groupcompress/groupcompress.py", line 624, in get_group
    return locations[key][0]
TypeError: 'NoneType' object is unsubscriptable

bzr 1.13dev on python 2.5.2 (linux2)
arguments: ['/home/ian/bin/bzr-brisbane', 'upgrade', '--gc-plain-chk255']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_AU.UTF-8'
plugins:
  bzrtools /home/ian/.bazaar/plugins/bzrtools [1.12]
  fastimport /home/ian/.bazaar/plugins/fastimport [0.8dev]
  groupcompress /home/ian/.bazaar/plugins/groupcompress [unknown]
  launchpad /home/ian/bzr/repo/brisbane-core/bzrlib/plugins/launchpad [unknown]
  netrc_credential_store /home/ian/bzr/repo/brisbane-core/bzrlib/plugins/netrc_credential_store [unknown]
  usertest /home/ian/.bazaar/plugins/usertest [unknown]
*** 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.

real 0m37.299s
user 0m15.265s
sys 0m2.248s
ian@humpback:~/scm-benchmark/scratch/emacs-chk$