CAN YOU SOLVE THIS RIDDLE IN 2 MINS?

Robert wants to enter in a private club but he doesn't know the password which is required.

He waits there and listens to conversations.

A club memeber comes at the door, and the bouncer says, "Twelve"
She replies, "Six" and the bouncer let her in.

Another memeber comes at the door and the bouncer says, "Six"
He replies "Three" and the bouncer let him in.

Robert thinks he has heard enough and walked up to the door.

Bouncer says, "Ten" and Robert replies, "Five" but the bouncer doesn't let him go in.

Why?