python tools/generate_docs.py -o doc/en/user-reference/index.txt rstx fails with a traceback in a fresh branch

Weird, I encounter the following when running 'make docs-sphinx' in a *fresh* bzr branch (don't even dream to reproduce it otherwise):

Traceback (most recent call last):
  File "tools/generate_docs.py", line 108, in <module>
    main(sys.argv)
  File "tools/generate_docs.py", line 94, in main
    infogen_mod.infogen(options, outfile)
  File "bzrlib/doc_generate/autodoc_rstx.py", line 56, in infogen
    outfile.write(_get_body(params, topic_dir))
  File "bzrlib/doc_generate/autodoc_rstx.py", line 68, in _get_body
    output_dir=topic_dir))
  File "bzrlib/doc_generate/autodoc_rstx.py", line 87, in _get_section
    help = registry.get_detail(topic)
  File "bzrlib/help_topics/__init__.py", line 92, in get_detail
    return obj(topic)
  File "bzrlib/help_topics/__init__.py", line 744, in get_current_formats_topic
    controldir.format_registry.help_topic(topic)
  File "bzrlib/controldir.py", line 1353, in help_topic
    default_help = self.get_help('default')
  File "bzrlib/registry.py", line 209, in get_help
    the_help = self._help_dict[self._get_key_or_default(key)]
KeyError: 'default'

The culprit command is 'python tools/generate_docs.py -o doc/en/user-reference/index.txt rstx'.

Adding 'from bzrlib import [workingtree|branch|repository]' is enough to fix the issue...