testfor with score parameters seems broken

Testfor commands which involve testing for players that fit within certain score parameters seem to be completely broken for me on this update. I have tested this across multiple worlds and multiple different scoreboards all with the same result.

For example, on one world where I have a scoreboard named "B" the following commands when entered into a regular impulse command block and a lever pulled give an error message.

testfor @p[score_B=0,score_B_min=0]
testfor @p[score_B=0]

The error is as follows "[09:51:15] The entity UUID provided is in an invalid format".

This error is given no matter what score range I give and no matter whether I fall within that range or not. Conversely if I change the @p to @a then no output is given by the command block at all, regardless of whether I fall within the score range or not.

It should be noted that commands such as testfor @p and testfor @p[r=5] work as expected.

Apologies in advance if this transpires to be user error, but I have tried doing these exact commands on Minecraft 1.8 with success.