Placing a command block with comparator and redstone in front of it don't make the redstone glow

• Details:
  Try Enter your world in Minecraft then try place a command block on the ground then place a comparator in front of it then place redstone in front of the comparator then type in the command block /testfor @p then you will find that the redstone doesn't glow but, by placing a redstone block and place a comparator in front of it then place redstone in front of the comparator you will find that redstone glow.

• How to reproduce:
  When placing a command block on the ground and place a comparator in front of it then place redstone in front of the comparator the redstone should glow.

Please fix that fast as you can bec. That confuse all the players.