ColorMask 0 - unsupported on DX11?

I think I've tracked down my issue to using ColorMask 0. If I use ColorMask 0 in a shader in DX11, deferred rendering, it renders pink pixels, like a broken shader. Although it does compile just fine. I can't find any mention on ColorMask not being supported in DX11/deferred, and it works as expected in DX9.

One thing that concerns me is that materials are created with the Standard shader by default, and the UI editor script that controls the material/shader makes changes to the material in code. Since it's the default material, these changes are always made. I may try resetting the values that UI script makes and see if that helps at all.

Any insight is much appreciated!

-Jamie