Poco::Nullable and std::optional assignement

[carculator]

I´m confused why this works not as expected:

        Poco::Nullable<std::string> src;
        src = "Test";
	std::cout << src.value() << std::endl; // output ist 'Test'

        std::optional< std::string> trg; 
        trg = src;
	std::cout << (trg.has_value() ? trg.value() : "null !?")  << std::endl; // output is 'null !?'

Can anyone help me out since from intuition this should work !