A VSX based add-in developed in Visual Studio 2010 returns a runtime error when hosted by Visual Studio 2013

I have written a Visual Studio Package based add-in for Visual Studio 2010.  I works perfectly when run within Visual Studio 2010.

I would like to migrate it to Visual Studio 2013, but am getting a run time error when I try to use the add-in in Visual Studio 2013.

The statement "VCProj = (VCProject)proj.Object" works in visual studio 2010 but throws a System.InvalidCastException in visual studio 2013.

To be clear, this is a runtime error.  Both Visual Studio 2010 and 2013 compile the VSX project without error.

VCProject's fully qualified namespace is "Microsoft.VisualStudio.VCProjectEngine.VCProject."
the "proj" object's fully qualified type is "EnvDTE.Project."

This project is a C# project, a visual studio package to be specific.  It is a C++ App (creating) Wizard that works perfectly when hosted by Visual Studio 2010.

Any helpful advice you can provide will be appreciated.

Charles S. Cotton