I can't run '/testfor' commands

I can't run '/testfor' commands in a command block or in chat.
Example in a commandblock: testfor @a

{SelectedItemSlot:1}

This was the output of the command block: [21:52:29] Unknown command. Try /help for a list of commands

What I expected to happen was...:
In my case there was a comparator against the command block, so there should have been an output (a redstone signal).

What actually happened was...:
It said that there was no command like that; '[21:52:29] Unknown command. Try /help for a list of commands'
And it did not give me an output/redstone signal.

Steps to Reproduce:
1. Place command block
2. Write a testfor command down, example: testfor @a

{SelectedItemSlot:1}

3. Place a comparator next to/against/behind the command block
4. Power the command block
5. See that it does't work and it says that it's an unknown command.