@p arguments not working with cubic areas.

So I decided to try and make an adventure map with the new snapshot, 14w03b and ran into a situation where I need a superfast clock and a command block to search for a player. I thought it may be convenient to use the new cubic areas that Searge previewed on his twitter.
I tried this out, and as you can see in the screenshots below, it kept giving me the error of "The entity UUID is in an invalid format." I understand what this means, but I have no idea of how it could happen.
I also tried just using a radius, and that worked. I also tried exact coordinates and relative coordinates and neither worked. I even tried using @a and @r, and these picked me up, and sent out a signal
I'm sort of a noob at command blocks and target selectors, so sorry if it's just a noobish mistake. Thanks for your help in advance, though.