Error: operator() will not be called for implicit or explicit conversions

So, I get the following error when compiling a class template.

vectors.h(441): error #597: "Vec3<T>::operator Vec3<T>()" will not be called for implicit or explicit conversions

The offending line is a casting operator for a 3-component vector class:

    operator Vec3() 
    {    return Vec3<double>(x, y, z);                                }

Vec3 is templated on the type of the scalar.

Has anyone come across this and is there a list that tabulates these error codes?