Unable to refactor (move) interface Follow

I'm trying to refactor (move) an interface (F6). I select the target folder but the Stop Sign appears with "Target project language should be C#". The target project language *is* C#. The source and target projects are both .NET 4.5. My solution contains projects create using both VS 2012 and VS 2015. The stop sign disappears if I choose a target project created using VS 2012 but appears for projects created using VS 2015.