>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='a')
  ...
  File ".../Lib/argparse.py", line 1333, in add_argument
    raise ValueError("length of metavar tuple does not match nargs")
ValueError: length of metavar tuple does not match nargs

The message should really be about nargs not having a valid value.  The nargs value is invalid regardless of the metavar.  There is also this:

>>> parser.add_argument('foo', nargs=-1)
_StoreAction(option_strings=[], dest='foo', nargs=-1, const=None, default=None, type=None, choices=None, help=None, metavar=None)

which is not consistent with this:

>>> parser.add_argument('foo', nargs=0)
  ...
    raise ValueError('nargs for store actions must be > 0; if you '
ValueError: nargs for store actions must be > 0; if you have nothing to store, actions such as store true or store const may be more appropriate