How to reference project from UnitTest project in a mixed DNX / .NET VS2015 solution?

My solution consists of an API project and 3 class libraries. I have a unit test project that I created at the same time as my solution.

I had some struggles with getting my DNX 4.5.1 API project to reference my .NET 4.5.1 class library projects. I managed to get rid of my referencing errors by first building each class library project to target DNX, then changing them to target .NET.

I now want to create unit tests for classes in my class libraries. However, when I try to reference the class libraries from the UnitTests project, I get an error dialog, text and screenshot below. There is no other information as to what could be causing this error. What am I doing wrong here?

A reference to '[project name]' could not be added.