Stencil drawing depends on color data?

I’m working on implementing stencil shadows in my project, and I have them working normally, but I’d like to use a vertex shader. Right now, I’ve developed a “passthrough” shader that simply transforms the vert, and I am doing the vertex projection myself (not in the shader).

The problem is, the shadow volume polys do not get rasterized to the stencil buffer unless I add a “MOV result.color, vertex.color” instruction to my vertex shader. This doesn’t make sense to me, because I have color writes disabled, as well as GL_BLEND and GL_ALPHA_TEST disabled. Why would having a color matter here?

Thanks in advance!