How do I stabilize soldiers?

So I've got a critically wounded soldier lying around. My medic has moved into range, but the med-pack icon remains greayed out. However, I can still select the med-pack and target the soldier, and the tooltip tells me to stabilize the target. However, pressing "Ok" does nothing. I've had this exact situation twice, in different missions. Is that a bug?

There is actually a separate skill for reviving downed members. It's a different icon than the heal skill (and if your skill bars are anything like mine, it's way on the other end, and it moves around from time to time). And yes, you do need a medpack charge available for use.