svn-upgrade fails with a KeyError (in rebase generate_transpose_plan)

I'm trying to see if svn-upgrade on a v3 branch would result in the same thing as a fresh v4 branch.

> bzr-experiment svn-upgrade
Using saved location: svn+ssh://source/home/wouter/tmp/kmx_svn/trunk/kmx
wouter@source's password:
bzr: ERROR: exceptions.KeyError: 'svn-v4:cd2972fe-eb09-0410-94ea-d8feb256cc61:trunk/kmx:8048'to fetch:copying revision:determining revisions to fetch:copying revision:determining revisions to fetch:copying revis

Traceback (most recent call last):
  File "/home/wouter/src/bzr/bzr.dev/bzrlib/commands.py", line 893, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/wouter/src/bzr/bzr.dev/bzrlib/commands.py", line 839, in run_bzr
    ret = run(*run_argv)
  File "/home/wouter/src/bzr/bzr.dev/bzrlib/commands.py", line 539, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/wouter/src/bzr/bzr.dev/bzrlib/commands.py", line 853, in ignore_pipe
    result = func(*args, **kwargs)
  File "/home/wouter/.bazaar/plugins/experiment/svn/__init__.py", line 348, in run
    allow_changes=True, verbose=verbose)
  File "/home/wouter/.bazaar/plugins/experiment/svn/foreign/upgrade.py", line 117, in upgrade_workingtree
    allow_changes=allow_changes, verbose=verbose)
  File "/home/wouter/.bazaar/plugins/experiment/svn/foreign/upgrade.py", line 165, in upgrade_branch
    allow_changes=allow_changes, verbose=verbose)
  File "/home/wouter/.bazaar/plugins/experiment/svn/foreign/upgrade.py", line 287, in upgrade_repository
    allow_changes=allow_changes)
  File "/home/wouter/.bazaar/plugins/experiment/svn/foreign/upgrade.py", line 253, in create_upgrade_plan
    graph, lambda revid: create_upgraded_revid(revid, new_mapping.upgrade_suffix))
  File "/home/wouter/.bazaar/plugins/experiment/rebase/rebase.py", line 226, in generate_transpose_plan
    replace_map[r] = (v, parent_map[v])
KeyError: 'svn-v4:cd2972fe-eb09-0410-94ea-d8feb256cc61:trunk/kmx:8048'

bzr 1.12dev on python 2.5.2 (linux2)
arguments: ['/home/wouter/src/bzr/bzr.dev/bzr', 'svn-upgrade']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'
plugins:
  bzrtools /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [1.6]
  launchpad /home/wouter/src/bzr/bzr.dev/bzrlib/plugins/launchpad [unknown]
  netrc_credential_store /home/wouter/src/bzr/bzr.dev/bzrlib/plugins/netrc_credential_store [unknown]
  rebase /home/wouter/.bazaar/plugins/experiment/rebase [0.4.3dev]
  svn /home/wouter/.bazaar/plugins/experiment/svn [0.5rc1]
*** 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.