Error during Bind10 Git repository mirroring

I am seeing the following error on https://code.launchpad.net/~vzeman79/bind10-mirror/bind10-master mirroring. Initial mirror request always succeeds but follow up updates always fail. See http://launchpadlibrarian.net/111062451/vzeman79-bind10-mirror-bind10-master.log:

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2012-07-24 13:59:32 INFO Starting job.
2012-07-24 13:59:32 INFO Getting exising bzr branch from central store.
2012-07-24 13:59:32 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2012-07-24 13:59:32 INFO 35 bytes transferred
2012-07-24 13:59:36 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2012-07-24 13:59:40 INFO Importing branch.
2012-07-24 13:59:43 INFO
Traceback (most recent call last):
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/scripts/code-import-worker.py", line 99, in <module>
    sys.exit(script.main())
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/scripts/code-import-worker.py", line 94, in main
    return import_worker.run()
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/lp/codehosting/codeimport/worker.py", line 591, in run
    return self._doImport()
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/lp/codehosting/codeimport/worker.py", line 745, in _doImport
    inter_branch.fetch(limit=revision_limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/branch.py", line 709, in fetch
    self.fetch_objects(stop_revision, fetch_tags=fetch_tags, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/branch.py", line 732, in fetch_objects
    determine_wants, self.source.mapping, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/fetch.py", line 705, in fetch_objects
    progress=lambda text: report_git_progress(pb, text))
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/remote.py", line 422, in fetch_objects
    lambda x: os.write(fd, x), progress)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/remote.py", line 412, in fetch_pack
    pack_data, progress)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/remote.py", line 226, in fetch_pack
    graph_walker, pack_data, progress)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/dulwich/client.py", line 462, in fetch_pack
    graph_walker, wants, can_read)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/dulwich/client.py", line 350, in _handle_upload_pack_head
    have = graph_walker.next()
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/dulwich/object_store.py", line 875, in next
    ps = self.get_parents(ret)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/lib/dulwich/object_store.py", line 197, in <lambda>
    return ObjectStoreGraphWalker(heads, lambda sha: self[sha].parents)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/object_store.py", line 657, in __getitem__
    _check_expected_sha(sha, commit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-15668/bzrplugins/git/object_store.py", line 149, in _check_expected_sha
    expected_sha))
AssertionError: Invalid sha for <Commit 340d1d72bf79947068a4899ee0330ef13a48d6f0>: 0d4685b3e7603585afde1b587cbfefdfaf6a1bb3
Import failed:
Traceback (most recent call last):
Failure: twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended with exit code 1.
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