"AttributeError: 'str' object has no attribute 'set_revision_history'" when pushing

[William Grant]

PQM on praseodymium had a very odd error this morning. It only happened once, but caused the push to fail.

Sep 29 19:16:09 pqm [139941724559104] INFO: current cwd is /home/pqm/pqm-queue/launchpad
Sep 29 19:16:09 pqm [139941724559104] INFO: getting working dir for /home/pqm/archives/rocketfuel/launchpad/db-devel
Sep 29 19:16:20 pqm [139941724559104] INFO: Executing star-merge bzr+ssh://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable at Thu Sep 29 19:16:20 2011
Sep 29 19:16:30 pqm [139941724559104] INFO: running precommit hook: [ -z "$(bzr ls -0 --versioned | xargs -0 grep '^\(<<<<<<< TREE\|>>>>>>> MERGE-SOURCE\)$')"
 ]
Sep 29 19:16:31 pqm [139941724559104] INFO: success: merge bzr+ssh://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable bzr+ssh://bazaar.launchpad.net/~laun
chpad-pqm/launchpad/db-devel
Sep 29 19:16:33 pqm [139941724559104] INFO: commit succeeded
Sep 29 19:16:33 pqm [139941724559104] INFO: publishing to 'bzr+ssh://bazaar.launchpad.net/~launchpad-pqm/launchpad/db-devel'
Sep 29 19:16:54 pqm [139941724559104] ERROR: Exception handling merge
Traceback (most recent call last):
  File "/home/pqm/pqm/pqm/script.py", line 711, in run
    line=line, output_obj=output_obj)
  File "/home/pqm/pqm/pqm/script.py", line 406, in do_merge
    output_obj, dir, self.commitmsg, to_repo_revision, config)
  File "/home/pqm/pqm/pqm/script.py", line 626, in wrap_command
    mangled_lines = command(sender, output_obj, *args)
  File "/home/pqm/pqm/pqm/__init__.py", line 419, in commit
    parent_branch.set_revision_history(
AttributeError: 'str' object has no attribute 'set_revision_history'