Creating a new package fails

The manual says that following commands should work:

$ bzr init scruff
$ cd scruff/
$ bzr merge-upstream ../scruff-0.1.tar.gz --version 0.1 \
    --distribution debian --package scruff

However an assertion fails when bootstrapping packaging project, i.e. creating a new package.

import_dsc.py has following assertion
1490        assert isinstance(previous_version, str), \
 1491            "Should pass str not %s" % str(type(previous_version))

bzr: ERROR: exceptions.AssertionError: Should pass previous_version as str not <type 'NoneType'>

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 917, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 1117, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 691, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 710, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/builddeb/cmds.py", line 714, in run
    merge_type, force)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/builddeb/cmds.py", line 550, in _do_merge
    merge_type=merge_type, force=force)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/builddeb/import_dsc.py", line 1493, in merge_upstream
    type(previous_version))
AssertionError: Should pass previous_version as str not <type 'NoneType'>