Compiler complains that I'm not passing it the right types, when I should be

I'm using rapidxml to parse some data, generate an xml document, and write it to a file. My code looks something like this:

#include <fstream>
#include "rapidxml_print.hpp"

int main()
{
    std::ifstream file;
    file.open(filename);
    file << (object.xml_generating_function());
}

The prototype for function_creating_xml is:

rapidxml::xml_document<> MyClass::xml_generating_function();

And the operator overload for << is defined in rapidxml_print.hpp. Yet g++ won't compile for me, first showing this:

main.cpp: In function ‘int main(int, char**)’:
main.cpp:82:35: error: no match for ‘operator<<’ in ‘file << object.xml_generating_function()()’
main.cpp:82:35: note: candidates are:

And lists all the standard definitions for the operator, followed curiously by this:

rapidxml_print.hpp:412:36: note: template<class Ch> std::basic_ostream<Ch>& rapidxml::operator<<(std::basic_ostream<Ch>&, const rapidxml::xml_node<Ch>&)

It would seem I'm not passing it the right argument times, but how could this be? I'm passing it an output stream and an xml document, the types it demands.