[boost] [voronoi] Issue with boost::polygon::voronoi_diagram<> as class variable

For some reason when I have a voronoi_diagram as a class variable (which it needs to be in this case), I get this error on compile: Error 49 error C2248: 'boost::polygon::voronoi_diagram<T>::operator =' : cannot access private member declared in class 'boost::polygon::voronoi_diagram<T>' I'm new to C# so it may be something obvious. - George