Command Block "testfor" command bug

I added a new objective, "fk", to the scoreboard. The "Counter" was totalKillCount. I also added a new team, called "fire". After that, I needed to use testfor, so that I can get rewarded when I reach the Required amount, which was 5. The command I put in was:

"testfor @p[[score_fk_min=5,team=fire]]" without quotes.

In the output box of the command block, it said "[[09:39:53]] The entity UUID provided is in an invalid format". This makes no sense, is it a bug?

Also, If you see 2 brackets for each command, it is because my computer thinks that it is HTML code, so I have to add in an Extra Bracket to each Side