"Could not load file or assembly" when running unit tests against ASP.NET MVC project

I have "Could not load file or assembly" error even the assembly exists (see attached screen shot)

The solution just contains 2 projects: a ASP.NET MVC 3 and a Class Library (for unit tests). The MVC project has no dependencies with third-party libraries. The unit test project only depends on NUnit and the MVC project.

There are 2 build steps: One uses Visual Studio solution build runner, while the other uses NUnit runner.

I tested on both TeamCity 6.0.2 and latest TeamCity 6.5 EAP build.

Anybody have same problem?