Attached is a patch which solves these problems and adds test cases of Chris Jerdonek. When nargs is negative the same ValueError is raised as when nargs is zero. Also when nargs is any other invalid value a ValueError("invalid value for nargs") is raised.

I have one question and that is about the value PARSER="A..." for nargs, it's a valid option and there are test cases with nargs="A...", however it is not listed in the documentation: http://docs.python.org/dev/library/argparse.html#nargs