Error while running VS tests

Question

I've bumped into a very strange problem lately with my friend while using unit test framework in Visual Studio 2010. We have created TestProject (let's call it NoTitleTests) that references few libraries ("casual" and "xna" dlls) and it works without any problem.
But when we added NoTitle project (which is a XNA game) to references, when we launch our units tests, VS comes up with error window:

"This interface is not supported. 
(Exception from HRESULT:0x80004002 (E_NOINTERFACE))"

(may differ a bit on english Windows, I've translated).

Funny thing is, that we got this error even if our unit test project contains zero tests. It compiles fine (as no code is in project, it would be hard to not compile :-) ).