error: 'struct std::complex<double>' has no member named 'Jsonize'

#include <Core/Core.h>

using namespace Upp;

template <class T>
void Jsonize(JsonIO& io, std::complex<T>& var) {
	T re, im;
	if (io.IsStoring()) {
		re = var.real();	
		im = var.imag();
	}
	io("re", re)("im", im);
	if (io.IsLoading()) {
		var.real(re);
		var.imag(im);
	}
}

struct Demo {
	std::complex<double> val;
	void Jsonize(JsonIO& io) 	{io("val", val);}
};

CONSOLE_APP_MAIN
{
	Demo demo;
	demo.val =  std::complex<double>(1, 2);
	StoreAsJsonFile(demo, "filename.json");
}