Mupen64 Win7, C-up and down bound to buttons don't work

This is really weird, but if I bind C-up and C-down to buttons, they don’t work. C-left and right do work though. I have to bind them using the C Buttons X and Y axes, because for some reason the C1 and C4 assignments do nothing, and C2 and C3 are also the A and B buttons, which makes no sense. It’s not a controller issue because it doesn’t work with keyboard either.