bzr send --format=git raises AttributeError: 'NoneType' object has no attribute 'splitlines'

Seen in the daily ppa.

In a branch of lp:cortex-strings, running bzr send --format=git shows this backtrace:

michaelh@crucis:~/linaro/minor/cortex-strings$ bzr send --format=git
Using saved parent location "bzr+ssh://bazaar.launchpad.net/%2Bbranch/cortex-strings/" to determine what changes to submit.
bzr: ERROR: exceptions.AttributeError: 'NoneType' object has no attribute 'splitlines'

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 930, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1130, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 691, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 713, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/builtins.py", line 5391, in run
    strict=strict)
  File "/usr/lib/python2.7/dist-packages/bzrlib/send.py", line 130, in send
    branch, tree)
  File "/usr/lib/python2.7/dist-packages/bzrlib/merge_directive.py", line 344, in compose_merge_request
    ''.join(self.to_lines()),
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/git/send.py", line 120, in to_lines
    return self.patch.splitlines(True)
AttributeError: 'NoneType' object has no attribute 'splitlines'

You can report this problem to Bazaar's developers by running
    apport-bug /var/crash/bzr.1000.2011-05-30T22:54.crash
if a bug-reporting window does not automatically appear.

This is on natty with:

michaelh@crucis:~/linaro/minor/cortex-strings$ dpkg -l|grep " bzr"
ii bzr 2.4.0~bzr5925~ppa3932.3931~natty1
ii bzr-builddeb 2.7.4+bzr561~natty1
ii bzr-git 0.5.4+bzr1270~ppa85~natty1
ii bzrtools 2.4.0~bzr767~ppa716~natty1