Serializing a map of objects to xml using boost::serialization

The serialization example below is from the boost mailing list which is pretty much the same as what I would like to do. However, I have changed the archive so that it will serialize to XML. The compile does not fail if I serialize to binary, but it fails when serializing to xml. The compile fails in basic_xml_oarchive.hpp in the following method:

// boost code where compile fails
template<class T>
void save_override(T & t, BOOST_PFTO int)
{
    // If your program fails to compile here, its most likely due to
    // not specifying an nvp wrapper around the variable to
    // be serialized.
    BOOST_MPL_ASSERT((serialization::is_wrapper<T>));
    this->detail_common_oarchive::save_override(t, 0);
}

It seems I haven't done enough to allow the std::map<int, CSomeData> object to be serialized, any ideas on how to fix this?

My serialization implementation:

#include <boost/archive/xml_oarchive.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/serialization/map.hpp>
#include <fstream>
#include <string>
#include <map>

using namespace std;

// This is a test class to use as the map data.
class CSomeData {
    public:
        CSomeData(){};
        CSomeData(float f0, string str0)
        {
            m_f0 = f0;
            m_str0 = str0;
        }

        float m_f0;
        string m_str0;

    private:
        friend class boost::serialization::access;

        template<class Archive>
        void serialize(Archive &ar, const unsigned int version)
        {
            ar & m_f0;
            ar & m_str0;
        }
};

// This is the class we really want to try serializing.
class CTest {
    public:
        CTest(){};
        CTest(int nNumber)
        {
            m_nNumber = nNumber;

            // Fill with some dummy data.
            m_mTst.insert(make_pair(0, CSomeData(0.23f, "hi hi hi")));
            m_mTst.insert(make_pair(1, CSomeData(7.65f, "second one")));
            m_mTst.insert(make_pair(2, CSomeData(9.23f, "third one")));
            m_mTst.insert(make_pair(3, CSomeData(5.6766, "chosen one")));
        }
        ~CTest(){};

        save()
        {
            std::ofstream ofs("filename");

            // Write class instance to archive. Writing seems to work ok.
            boost::archive::xml_oarchive oa(ofs);
            oa << BOOST_SERIALIZATION_NVP(*this);
        }

        int m_nNumber;

    private:
        map<int, CSomeData> m_mTst;

        friend class boost::serialization::access;

        template<class Archive>
        void serialize(Archive &ar, const unsigned int version)
        {
            ar & m_nNumber;
            ar & m_mTst;
        }
};