testforblocks Bug

I was making an adventure map, and i wanted to check if the player cleared the board. For this i used the /testforblocks command, but when I cleared the board, it gave no output, i tried testing for the same area, comapring exact blocks. using the command /testforblocks 984 61 614 994 61 604 984 61 614 994 61 604 and it gives no output