assertion error segfault when branching from svn+http://svn.effbot.python-hosting.com

I was trying to branch a Subversion repo, and got an assertion error from subversion:

  $ bzr branch -Dtransport svn+http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge sf-import
  python: /build/buildd/subversion-1.4.3dfsg1/subversion/libsvn_subr/path.c:377: svn_path_basename: Assertion `is_canonical(path, len)' failed.
  Aborted (core dumped)

The corresponding ~/.bzr.log output was:

  bzr arguments: [u'branch', u'-Dtransport', u'svn+http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge', u'sf-import']
  looking for plugins in /home/james/.bazaar/plugins
  looking for plugins in /usr/lib/python2.5/site-packages/bzrlib/plugins
  Plugin name __init__ already loaded
  Plugin name __init__ already loaded
  encoding stdout as sys.stdout encoding 'UTF-8'
  opening SVN RA connection to 'http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge'
  svn get-repos-root
  opening SVN RA connection to 'http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge'
  svn get-uuid
  svn get-latest-revnum
  request path 'stuff/sandbox/sourceforge' -> 'http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge'
  svn check_path -r458 http://svn.effbot.python-hosting.com/stuff/sandbox/sourceforge

This was using bzr-0.90~rc1-1 (haven't gotten round to updating) with r687 of http://people.samba.org/bzr/jelmer/bzr-svn/0.4/. I am on an Ubuntu Feisty AMD64 system with python-subversion 1.4.3dfsg1-1ubuntu1.

Doing a bit of post-mortem debugging with gdb, the Python stack appears to be:

  /var/lib/python-support/python2.5/libsvn/ra.py (480): svn_ra_check_path
  /home/james/.bazaar/plugins/svn/transport.py (415): check_path
  /home/james/.bazaar/plugins/svn/transport.py (82): convert
  /home/james/.bazaar/plugins/svn/errors.py (74): convert
  /home/james/.bazaar/plugins/svn/branch.py (79): __init__
  /home/james/.bazaar/plugins/svn/format.py (179): open_branch
  /usr/lib/python2.5/site-packages/bzrlib/branch.py (132): open
  /usr/lib/python2.5/site-packages/bzrlib/builtins.py (858): run
  /usr/lib/python2.5/site-packages/bzrlib/commands.py (477): run_argv_aliases
  /usr/lib/python2.5/site-packages/bzrlib/commands.py (780): run_bzr
  /usr/lib/python2.5/site-packages/bzrlib/commands.py (780): run_bzr_catch_errors
  /usr/lib/python2.5/site-packages/bzrlib/commands.py (818): main
  /usr/bin/bzr (811): <module>