How do I specify a default value for vector<string> in boost::program_options

I do want to give a default value for the positional parameter as in the comment in the code, but the compiler complains. The code as it is compiles fine. I use boost 1.46.1 and g++

int main(int argc, char *argv[]) {
    namespace po = boost::program_options;

    po::positional_options_description p;
    p.add("path", -1);

    po::options_description desc("Options");
    std::vector<std::string> vec_str;
    std::string str;
    desc.add_options()
        ("foo,f", po::value< std::string >()->default_value(str), "bar")
        //("path,p", po::value< std::vector<std::string> >()->default_value(vec_str), "input files.")
        ("path,p", po::value< std::vector<std::string> >(), "input files.")
    ;

    po::variables_map vm;
    po::store(po::command_line_parser(argc, argv).options(desc).positional(p).run(), vm);
    po::notify(vm);
}