branching twisted svn fails with "bzr: ERROR: The branch svn://svn.twistedmatrix.com/svn/Twisted/trunk has no revision None."

Hacking bzr to not hide the traceback from us gives:

bzr: ERROR: bzrlib.errors.NoSuchRevision: KnitPackRepository('file:///home/mwh/src/twisted-bzr-svn/.bzr/repository/') has no revision ('<email address hidden>',)

Traceback (most recent call last):
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/commands.py", line 893, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/commands.py", line 839, in run_bzr
    ret = run(*run_argv)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/commands.py", line 539, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/builtins.py", line 980, in run
    force_new_repo=standalone)
  File "/home/mwh/.bazaar/plugins-disabled/svn/remote.py", line 64, in sprout
    return super(SvnRemoteAccess, self).sprout(*args, **kwargs)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/bzrdir.py", line 1113, in sprout
    result_repo.fetch(source_repository, revision_id=revision_id)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/repository.py", line 989, in fetch
    find_ghosts=find_ghosts)
  File "/home/mwh/.bazaar/plugins-disabled/svn/fetch.py", line 822, in fetch
    self._fetch_switch(self.source.transport.get_svn_repos_root(), needed, pb)
  File "/home/mwh/.bazaar/plugins-disabled/svn/fetch.py", line 734, in _fetch_switch
    parent_inv = self.target.get_inventory(parent_revid)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/repository.py", line 1534, in get_inventory
    return self.iter_inventories([revision_id]).next()
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/repository.py", line 1552, in _iter_inventories
    for text, revision_id in self._iter_inventory_xmls(revision_ids):
  File "/home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/repository.py", line 1563, in _iter_inventory_xmls
    raise errors.NoSuchRevision(self, record.key)
NoSuchRevision: KnitPackRepository('file:///home/mwh/src/twisted-bzr-svn/.bzr/repository/') has no revision ('<email address hidden>',)

bzr 1.9dev on python 2.5.2 (linux2)
arguments: ['/home/mwh/canonical/repos/bzr/bzr.dev/bzr', 'get', 'svn://svn.twistedmatrix.com/svn/Twisted/trunk', 'twisted-bzr-svn']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_NZ.UTF-8'
plugins:
  gtk /usr/lib/python2.5/site-packages/bzrlib/plugins/gtk [0.95.0]
  launchpad /home/mwh/canonical/repos/bzr/bzr.dev/bzrlib/plugins/launchpad [unknown]
  loggerserve /home/mwh/.bazaar/plugins-disabled/loggerserve [unknown]
  pqm /usr/lib/python2.5/site-packages/bzrlib/plugins/pqm [1.0.0dev0]
  svn /home/mwh/.bazaar/plugins-disabled/svn [0.4.14dev0]
*** 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.

This is somehow related to roundtripped revisions in the repository.