def verify(parser, argv):
    """verify(parser, argv)
    Check for input errors

    Arguments:
        parser: OptionParser instance
        argv (list): Argument list

    Returns:
        An error message in the event of an input error, or None
    """
    opts, args = parser.parse_args(argv)
    err_msg = None
    if opts.script and len(args) != 1:
        err_msg = "Must provide a script\n"

    return err_msg