argparse optional positional argument and subparsers arguments

I have a python script that takes in an optional positional argument and has a few subcommands. Some of these subcommands require the positional argument, some don't. The problem I have appears when I try to use a subcommand that does not require the positional argument. Consider the following test file:

import argparse

argp = argparse.ArgumentParser()
argp.add_argument('inputfile', type=str, nargs='?',
                  help='input file to process')
argp.add_argument('--main_opt1', type=str,
                  help='global option')

subp = argp.add_subparsers(title='subcommands',
                           dest='parser_name',
                           help='additional help',
                           metavar="<command>")

tmpp = subp.add_parser('command1', help='command1 help')
tmpp.add_argument('pos_arg1', type=str,
                  help='positional argument')

print repr(argp.parse_args())

When I try to use the subcommand command1 with the first argument everything goes well.

macbook-pro:~ jmlopez$ python pytest.py filename command1 otherarg
Namespace(inputfile='filename', main_opt1=None, parser_name='command1', pos_arg1='otherarg')

But now let us assume that command1 doesn't need the first positional argument.

macbook-pro:~ jmlopez$ python pytest.py command1 otherarg
usage: pytest.py [-h] [--main_opt1 MAIN_OPT1] [inputfile] <command> ...
pytest.py: error: argument <command>: invalid choice: 'otherarg' (choose from 'command1')

I was somehow expecting inputfile to be set to None. Is there any way that argparse can predict that command1 is actually a subcommand and thus inputfile should be set to None?