When I place redstone on top of a command block it is invisible?

When i place a piece of redstone on top of a command block it is invisible!?

What I expected to happen was...:
I would think it would show a piece of redstone on top of the command block.

What actually happened was...:
the redstone just disappeared

Steps to Reproduce:
1. Get a command block .
2. Place on ground.
3. Place redstone on top.