An assembly must have a 'dll' extension in order to be referenced???

And how did I get such a useful error you might ask?  By trying to add a reference to a project that is in the same solution in VS.Net 2003.  The project I'm trying to reference is a class library and when I open the project by itself, it builds perfectly fine.

I've got 3 different class library projects added to this solution of mine and 2 of them added as references just fine, it's this 3rd one that's giving me trouble.  Can anyone offer a suggestion as to what to check for this?

I can workaround it for now by just adding the .dll itself, but I'd rather have the project in the same solution for ease of debugging...