Commands in command blocks not working correctly.

What I expected to happen was...:
I could type in /tp @p [x] [y] [z] to a command block and it teleported the closest player.

What actually happened was...:
It didn't when I sent a redstone signal and the last output was "please input the target player" or something like that, even though the target was @p.

Steps to Reproduce:
1. Place a command block
2. Type "/tp @p [coords]"
3. Send redstone signal