Using Depth Mask Shader to Cut hole though geometry Show Background

I'm Trying to make a masking shader to cut-out shapes in my geometry, and display the skybox behind them.

Here is an example of precisely what I want and a brief guide on how to achieve it: http://blog.polycrime.com/post/81848506548/how-to-cut-a-hole-through-geometry-to-the-skybox

Unfortunately, instead of the Skybox, it renders those pixels in black.

What could I be missing?