CSS checking breaks when "supports" is used

I checked a CSS file that validated on the W3C CSS validator, so I know there isn't an error in it. It worked when I ran it through the first time, as far as I could see, but when I wrapped one of the rules in a CSS "supports" block, the parser gave me an "unclosed block" error. I'm guessing it doesn't recognize "@supports" as the beginning of a block?