question to sudoers

Hi, I try to make one bash script to be run as root as a user without the password being asked for. As a test to see how this works, I edited sudoers with visudo and added this line at the end of the file: karl ALL= NOPASSWD: /bin/su After a restart I expected user karl to be able to run su without password. It didn't work, where's my mistake? thanks, Karl