"Path to custom Nice menus css file" is broken

I've used nice menus for years in D6. Now I'm building sites with D7 and have a problem pointing the nice menus module to a custom css file. It's simply not working. I copied the default_nice_menus.css file from the nice menus directory to my own theme, pointed to that file in my theme settings, cleared my caches a dozen times and no luck. The nice menus are properly styled when l leave the "Path to custom Nice menus cs file" blank. When i add a custom path to css, the nice menu displays as a standard list item instead of a nice menu. I've checked my syntax over and over again, I know it's pointing to the correct file.

This bug is also theme-independent. I've tried this on two different sites using Zen 7.x-3.1, Zen 7.x-5.1, and Bartik 7.12 and the result is the same every time: Nice menus cannot accept a path to custom css.