Rufus

Question: Rufus is a very rich man. He wants to guard a priceless jeweled necklace, so he locks it in a room and places a guard by the door. He comes up with a code so that only trusted people can get inside the room. A man walks up to the door and the guard says, "Twelve." and the man replies "Six." The man is let inside. another man walks up to the door and the guard says "Six." and the man says "Three." The man is also let inside. A thief, who has been watching closely, thinks he gets the code and walks up to the guard. The guard says "Eight." and the thief says "Four." The thief is immediately arrested. Why? What was the real code?