Vertex position in 2D shader always zero on Mac

I'm writing a 2D shader in Unity 5. In the vertex function, I'm doing the usual:

 OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);

In the fragment function, when I access the value

 IN.vertex.xy

It is always (0,0). I ran the same shader in the same version of Unity 5 on Windows, and it's giving me the expected values (I tested this by setting the return color in the fragment function to the vertex values).

Any ideas why this would be different on a Mac, and what I might be able to do about it?