Comparator clock not working with command blocks

I was messing around with command blocks when i found this bug.

Steps to reproduce:

1 - Set up a command block with a command:
I put: /testfor @p {SelectedItemSlot:0}

2 - Set up a classic comparator clock (1 comparator in subtract mode, 3 redstone dust around the comparator, 1 redstone block behind the comparator) going in to the command block

It should work right? No.
It emits an output only on the first redstone input, I tryed with a repeater clock too and it works fine.