4K 60Hz won't work

I'm running 10.12.6 on a GTX 960 with the Nvidia web drivers 378.05.05.25f01.

3840x2160@30Hz works fine, but 60Hz won't work on anything above 1920x1080. If I try to raise the resolution or refresh rate, all I get is a black screen.

Weirdly enough, my Monitors pane shows both "1920 x 1080" and "1080p" as different options. 1080p works, 1920x1080 gives me a black screen.