Test project reference Xamarin forms project breaks

If I create a bare bones Xamarin forms project, add an NUnit test project and then add a reference to the forms project from the test project, the forms project no longer builds.

It cannot find the Xamarin or Xamarin.Forms namespace. If I them remove the reference to the forms project from the test project, it works again.

Does anyone know why this is, or how I can have a unit test project that tests a Xamarin forms project?

Thanks!