def( str( self)) error

I have a class that uses an overloaded << and >> operators for I/O. 
When I try to export that class with the member function .def( str(
self)), I get a 'no function for call to' error.  My operator has the
following signature in C++, and works just fine for the standard
streams, and stringstreams:

namespace std 
{
template<typename char_T, class traits>
basic_ostream<char_T, traits>&
operator<<( basic_ostream<char_T, traits>& stream, const
physics::vector& v); // Definition not shown for brevity.
} // !namespace std

I'm using G++ 3.2.2, with identical errors on Win32/MinGW and Debian
Sid.

Thanks,
Jonathan

error quote ---------------
no matching function for call to boost::python::class_<physics::vector,
boost::python::detail::not_specified, (two more
times)>::def(boost::python::str)
----------- end quote-----