bzr patch --strip crash on non integer argument

using --strip with non relevant argument such as a character crash
here the stacktrace:
 bzr patch ~/Desktop/django-ticket#6845/6845-against-7424.diff --strip=a
bzr: ERROR: exceptions.ValueError: invalid literal for int() with base 10: 'a'

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 846, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 797, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools/command.py", line 13, in run_argv_aliases
    commands.Command.run_argv_aliases(self, argv, alias_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 472, in run_argv_aliases
    args, opts = parse_args(self, argv, alias_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 551, in parse_args
    options, args = parser.parse_args(args)
  File "/usr/lib/python2.5/optparse.py", line 1385, in parse_args
    stop = self._process_args(largs, rargs, values)
  File "/usr/lib/python2.5/optparse.py", line 1425, in _process_args
    self._process_long_opt(rargs, values)
  File "/usr/lib/python2.5/optparse.py", line 1500, in _process_long_opt
    option.process(opt, value, values, self)
  File "/usr/lib/python2.5/optparse.py", line 782, in process
    self.action, self.dest, opt, value, values, parser)
  File "/usr/lib/python2.5/optparse.py", line 802, in take_action
    self.callback(self, opt, value, parser, *args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/option.py", line 237, in _optparse_callback
    v = self.type(value)
ValueError: invalid literal for int() with base 10: 'a'

bzr 1.5 on python 2.5.2 (linux2)
arguments: ['/usr/bin/bzr', 'patch', '/home/visi/Desktop/django-ticket#6845/6845-against-7424.diff', '--strip=a']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'
plugins:
  avahi /home/visi/.bazaar/plugins/avahi [0.2.0]
  bisect /home/visi/.bazaar/plugins/bisect [1.1.0pre0]
  bzrtools /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [1.5.0]
  dbus /usr/lib/python2.5/site-packages/bzrlib/plugins/dbus [unknown]
  gtk /usr/lib/python2.5/site-packages/bzrlib/plugins/gtk [0.93.0]
  launchpad /usr/lib/python2.5/site-packages/bzrlib/plugins/launchpad [unknown]
  svn /home/visi/.bazaar/plugins/svn [0.4.11dev0]
  upload /home/visi/.bazaar/plugins/upload [0.1.0]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.