Enemy Isn't Hurting Player in Survival Shooter

Hello! I am trying to follow Unity's Survival Shooter tutorials, but can't get my enemy to hurt the player. It will follow, but won't hurt the player when he attacks. When I got to [phase 7][1] and uncommented the all the code, that's when my enemy stopped hurting the player. I also get this error: NullReferenceException: Object reference not set to an instance of an object PlayerHealth.Update () (at Assets/Scripts/Player/PlayerHealth.cs:38) How can I fix these problems? Thanks, saddlepiggy. [1]: https://www.youtube.com/watch?v=l86gpYbQFzY&index=7&list=PLX2vGYjWbI0SLoX3q1tUf3RJU_WzcLcHL