I recently made some unit tests for my Asp.NET MVC application, or I tried to, it keep telling me "The type 'System.Web.Mvc.Controller' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Mvc, Version=5.1.0.0, Culture=neutral.' Then I tried to add a reference to the above mentioned, but was only able to choose version 4.0.0.0 and now it tells me that my project already uses version 5.1.0.0, which is a higher version. What to do?

Thanks a bunch!