Tracks still play even though the bus they're routed through is muted.

I have three tracks routed through a bus -- i.e., the output of all three tracks is this bus. There are no sends. When I mute the bus, the tracks still play but at a lower volume. I've tried changing the bus number (just in case there was another bus with the same number), but that doesn't help. These tracks go to, and only to, this particular bus. How do I fix this?