empty leading and comment lines in debian/control break source package name finding

e.g. building gedit which has the attached control file, the following exception is raised:

bzr: ERROR: exceptions.KeyError: 'Source'

Traceback (most recent call last):
  File "/home/jelmer/src/bzr/foreign/bzrlib/commands.py", line 946, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/jelmer/src/bzr/foreign/bzrlib/commands.py", line 1150, in run_bzr
    ret = run(*run_argv)
  File "/home/jelmer/src/bzr/foreign/bzrlib/commands.py", line 699, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/jelmer/src/bzr/foreign/bzrlib/commands.py", line 721, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/jelmer/src/bzr/foreign/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/jelmer/src/bzr/foreign/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/jelmer/.bazaar/plugins/builder/cmds.py", line 701, in run
    package = debian_source_package_name(control_path)
  File "/home/jelmer/.bazaar/plugins/builder/cmds.py", line 549, in debian_source_package_name
    return control["Source"]
  File "/usr/lib/pymodules/python2.7/debian/deb822.py", line 175, in __getitem__
    value = self.__dict[key]
KeyError: 'Source'

You can report this problem to Bazaar's developers by running
    apport-bug /var/crash/bzr.1000.2011-07-01T10:40.crash
if a bug-reporting window does not automatically appear.

https://rt.admin.canonical.com//Ticket/Display.html?id=46345