[boost] program_options: Why it throw ambiguous_option?

Hi, I got the following error for the program list below. Where program_options considers "help" and "help_abc" as ambiguous? $ ./main --help terminate called after throwing an instance of 'boost::program_options::ambiguous_option' what(): ambiguous option help Aborted Thanks, Peng #include <boost/program_options.hpp> namespace po = boost::program_options; int main(int ac, char* av[]) { po::options_description desc("Allowed options"); desc.add_options() ("help", "produce help message") ("help_abc", po::value<int>(), "set compression level") ; po::variables_map vm; po::store(po::parse_command_line(ac, av, desc), vm); po::notify(vm); return 0; }