Question Uisng Texture Mask with lerp node to limit grass movement

hello, i am following along with this Brackeys tutorial on creating grass movement shader. at some point he uses uv coordinates of the mesh and lerp node to make sure the bottom of the grass stays on the ground, like this:

however when i want to use a texture mask instead of uv, the lerp node refuses to accept the output of the Sample texture 2d node (neither of the r, g or b values). why is this happening?