Minus front not working - Trying to create a half-circle line
I'm trying to take a circular path and cut it in half to create a half-circle line.
So, I drew a circular path, and then a straight line over it. I selected both, and then selected Minus Front from the Pathfinder panel, but when I do that the line just disappears and the circular path remains intact. It doesn't get split in half the way I was anticipating. If anyone can please tell me what I'm doing wrong here, that would be much appreciated!

