Reads first 2 bytes more than once

I got the following crash while trying to branch from the public git mirror of the Scipy project

$ bzr branch http://projects.scipy.org/git/scipy.git scipy-git-bzr
bzr: ERROR: exceptions.ValueError: need more than 0 values to unpack

Traceback (most recent call last):
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/commands.py", line 909, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/commands.py", line 1107, in run_bzr
    ret = run(*run_argv)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/commands.py", line 687, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/commands.py", line 702, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/cleanup.py", line 122, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/cleanup.py", line 156, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/builtins.py", line 1216, in run
    revision_id = br_from.last_revision()
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/decorators.py", line 140, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/Users/nmb/.bazaar/plugins/git/branch.py", line 250, in last_revision
    return self.lookup_foreign_revision_id(self.head)
  File "/Users/nmb/.bazaar/plugins/git/branch.py", line 258, in lookup_foreign_revision_id
    self.mapping)
  File "/Users/nmb/.bazaar/plugins/git/repository.py", line 172, in lookup_foreign_revision_id
    commit = self._git[foreign_revid]
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/repo.py", line 981, in __getitem__
    return self.object_store[name]
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/object_store.py", line 104, in __getitem__
    type_num, uncomp = self.get_raw(sha)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/object_store.py", line 312, in get_raw
    return ret.type_num, ret.as_raw_string()
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/objects.py", line 226, in as_raw_string
    return "".join(self.as_raw_chunks())
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/objects.py", line 220, in as_raw_chunks
    self._ensure_parsed()
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/objects.py", line 241, in _ensure_parsed
    self._parse_file(self._file)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/objects.py", line 326, in _parse_file
    if self._is_legacy_object(magic):
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.6.0-py2.6-macosx-10.3-fat.egg/dulwich/objects.py", line 290, in _is_legacy_object
    b0, b1 = map(ord, magic)
ValueError: need more than 0 values to unpack

bzr 2.2.0dev1 on python 2.6.5 (Darwin-9.8.0-i386-32bit)
arguments: ['/Users/nmb/usr/bin/bzr', 'branch', 'http://projects.scipy.org/git/scipy.git', 'scipy-git-bzr']
encoding: 'UTF-8', fsenc: 'utf-8', lang: 'en_US.UTF-8'
plugins:
  automirror /Users/nmb/.bazaar/plugins/automirror [0.1.0]
  bash_completion /Users/nmb/src/bzr/bzr.dev/bzrlib/plugins/bash_completion [2.2.0dev1]
  bzrtools /Users/nmb/.bazaar/plugins/bzrtools [2.2.0]
  colo /Users/nmb/.bazaar/plugins/colo [0.2.0dev]
  explorer /Users/nmb/.bazaar/plugins/explorer [1.1.0dev]
  fastimport /Users/nmb/.bazaar/plugins/fastimport [0.9.0dev]
  git /Users/nmb/.bazaar/plugins/git [0.5.1]
  hg /Users/nmb/.bazaar/plugins/hg [0.2.0dev]
  latex_version_info /Users/nmb/.bazaar/plugins/latex_version_info [0.0.2]
  launchpad /Users/nmb/src/bzr/bzr.dev/bzrlib/plugins/launchpad [2.2.0dev1]
  netrc_credential_store /Users/nmb/src/bzr/bzr.dev/bzrlib/plugins/netrc_credential_store [2.2.0dev1]
  news_merge /Users/nmb/src/bzr/bzr.dev/bzrlib/plugins/news_merge [2.2.0dev1]
  qbzr /Users/nmb/.bazaar/plugins/qbzr [0.19.0dev1]
  rst2html /Users/nmb/.bazaar/plugins/rst2html [0.0.2]

$ python -c "import dulwich; print dulwich.__version__"
(0, 6, 0)
$ python --version
Python 2.6.5