/testfor in commandblock

I was using /testfor @e[name=Justin_Time] for an adventure map I am making. It works if you manually type it in chat but it doesn't work in command blocks.

What I expected to happen was...:
It to give a comparator output.

What actually happened was...:
Unkown Command type /help for list of commands

Steps to Reproduce:
1. Type /testfor @e[Justin_Time] in command block
2. Unkown Command type /help for list of commands

Notice how this happens.
1. Type /testfor @e[Justin_Time] in chat
2. Found Justin_Time