Missing .dll, reference

Hello. I have Windows Phone 8.1 Project and I builded it with Unity. In Visual Studio on reference in Assembly-CSharp[...] to Assembly-CSharp-firstpass[...] there is a yellow triangle. "Path" property is empty and I can't find Assembly-CSharp-firstpass[...].dll in my project. Unity shouldn't create one? What can I do to fix that? Thank you.