I cannot detect scoreboard scores in 15w45a

I am trying to effect myself with levitation when I sneak using the stat.sneakTime scoreboard objective, but this repeating command block says that it 'an invalid entity UUID
my input was:
/testfor aether_hawk[score_Sneak_min=0] (This is actually probably the problem- I'm not the best at scoreboard.)
And the output was: [18:39:15] The entity UUID provided is in an invalid format
Can you please fix this soon as I is very irritating.
Thanks,
Aether_hawk