Spirit+Phoenix problem

#define BOOST_SPIRIT_USE_PHOENIX_V3 1

#include <string>
#include <iostream>
#include <boost/phoenix/core.hpp>
#include <boost/spirit/include/qi_core.hpp>
#include <boost/spirit/include/qi_lit.hpp>

namespace qi = boost::spirit::qi;
namespace phoenix = boost::phoenix;

bool parse_counter_placeholder(std::string::const_iterator& it, std::string::const_iterator end, unsigned int& width)
{
    return qi::parse
    (
        it, end,
        (
            qi::uint_[phoenix::ref(width) = qi::_1_type()]
        )
    );
}

int main(int argc, char* argv[])
{
    std::string str;
    if (argc >= 2)
        str = argv[1];
    else
        str = "5N";

    unsigned int width = 0;

    std::string::const_iterator it = str.begin();
    bool res = parse_counter_placeholder(it, str.end(), width);

    std::cout << "res = " << res << ", width = " << width << std::endl;

    return 0;
}