Can't display base interface in class diagram ("type not found")

I have a class diagram associated to a Silverlight class l开发者_StackOverflow社区ibrary project in Visual Studio 2008 SP1. One of the pieces of code displayed in the diagram is an interface I have made that inherits from System.Windows.Input.ICommand.

Now I want the base class of that interface (ICommand) to be shown in the diagram as well. So I right-click in the box representing the inherited interface, and select Display base interfaces. Then Visual Studio shows the following error:

Can't display one or more base types because the following type can't be found: System.Windows.Input.ICommand

The System.Windows assembly is properly referenced, the code for the inherited assembly has no errors, and the project compiles without problems.

So, what's happening here? Maybe a bug in Visual Studio?

To try to reproduce the error I have done the following (don't mind the naming of my objects):

• create an Interface (ICompany) and inherit from System.Windows.Input.ICommand
• create an entity (Company) that implements my interface
• create a class diagram and added my interface and class
• right clicked my interface and selected show interface

I'm using Visual Studio 2008 with the latest SP installed and I don't seem to get the error. Do I do something different then you?

Is your diagram located in your SL app/lib or in your Web app?