Spline Mask not working as expected

Hello,

I've been watching some very cool things being done with the spline mask tool and I wanted to give it a shot myself, but I've run into a problem I'm not seeing in any of the tutorials I've watched. I created my two spline objects and put them inside the spline mask, and then swithced it to " A subtract B " expecting one spline to cut a chunk out of the other, but instead all that happens is one spline will become invisible. I've tried moving them around and changing their stacking order but all that does is to change which one is being displayed. Can anyone tell me why the splines aren't masking as expected?

Thanks