Subtracting unclosed path in Sketch

I essentially want to subtract a dotted, unclosed path from a background shape in Sketch. I can see why this doesn't work, but I'm wondering if there's a way to do what I want to do (subtract the dotted shapes from the background shape). I think I'd have to expand the stroke (or in Sketch's case, border), but I'm not sure if that's possible.

Here it is before subtraction:

Here it is afterward (it is essentially closing the path before subtracting it)