Visual Studio 2010 Sequence Diagrams Cannot load

I was editing a VS 2010 Sequence Diagram and after saving and closing it, I can no longer open it. I get the following error. Anyone know how to resolve this?

"Canno开发者_C百科t Load '{diagram path and name}': Value cannot be null. Parameter name: rolePlayer"

The title of the dialog is "Microsoft Visual Studio"

This question was answered on the MSDN Visualization & Modeling Tools forum.