Can't plug Combiner Output into Anything

Hey All;
I’m a noob and still learning.
I have a combiner node that is at the end of a long chain. It is creating a black and white mask image that I wanted to use to blend between grass and dirt color.

How ever it does not allow me to plug it into a mask channel for a combiner. It says it has a bitmap output but I can’t even plug it into a bitmap output node.

Does anyone know how I might work around this?