Custom validator doesn't allow default_value

I am trying to use a custom validator instead of overloading operator>> to support an enum type in my option parsing. I have done the following:

#include <iostream>

#include <boost/program_options.hpp>

enum class MyEnum
{
    OneThing,
    AnotherThing
};

void validate(boost::any& v, const std::vector<std::string>& values,
              MyEnum*, int)
{
    // parse the enum from values[0]
    std::cout << "Test" << std::endl;
    MyEnum enumValue = MyEnum::OneThing;
    v = enumValue;
}

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

    po::options_description desc("Options");

    //desc.add_options()("myEnum", po::value<MyEnum>(), "MyEnum value"); // works fine
    desc.add_options()("myEnum", po::value<MyEnum>()->default_value(MyEnum::OneThing), "MyEnum value"); // compiler error: Source type is not streamable

    po::variables_map vm;
    po::store(po::parse_command_line(argc, argv, desc), vm);
    po::notify(vm);

    MyEnum myEnum = vm["myEnum"].as<MyEnum>();

    return 0;
}

This works fine as long as I don't try to set a default_value, but when I do specify a default_value, I get error: static assertion failed: Source type is neither std::ostream able nor std::wostream able. What else do I need (without overloading stream operators, which is the whole point of using a validator as far as I understand) to do to allow a custom type to get a default_value?