[pyar] mini problema con argparse

[Juan BC]

estoy migrando algunas cosas de optparse a argparse... y tengo siguiente 
código

#===============================================================
#coso.py
import argparse

parser = argparse.ArgumentParser()
parser.add_argument("command",
                     action="store",
                     type=str,
                     metavar="<COMMAND>")
parser.add_argument("options",
                     action="store",
                     nargs='*',
                     type=str,
                     default=[],
                     metavar="<OPTION, [OPTION_2, ..., OPTION_N]>")

parser.parse_args()
#===============================================================

al ejecutarlo me tira la siguiente exception

#===============================================================
   File "coso.py", line 15, in <module>
     parser.parse_args()
   File "/usr/lib/pymodules/python2.6/argparse.py", line 1698, in parse_args
     args, argv = self.parse_known_args(args, namespace)
   File "/usr/lib/pymodules/python2.6/argparse.py", line 1730, in 
parse_known_args
     return self._parse_known_args(args, namespace)
   File "/usr/lib/pymodules/python2.6/argparse.py", line 1943, in 
_parse_known_args
     self.error(_('too few arguments'))
   File "/usr/lib/pymodules/python2.6/argparse.py", line 2352, in error
     self.print_usage(_sys.stderr)
   File "/usr/lib/pymodules/python2.6/argparse.py", line 2314, in 
print_usage
     self._print_message(self.format_usage(), file)
   File "/usr/lib/pymodules/python2.6/argparse.py", line 2270, in 
format_usage
     return formatter.format_help()
   File "/usr/lib/pymodules/python2.6/argparse.py", line 314, in format_help
     help = self._root_section.format_help()
   File "/usr/lib/pymodules/python2.6/argparse.py", line 244, in format_help
     func(*args)
   File "/usr/lib/pymodules/python2.6/argparse.py", line 366, in 
_format_usage
     assert ' '.join(pos_parts) == pos_usage
AssertionError
#===============================================================

LA verdad que muy no tengo bien idea por que sucede y me la pase leyendo 
la documentación(seguro que se me paso algo por arriba)