NullReferenceException by detecting health

Hello everyone,
I’m working on a fps zombie game, currently I’m writing the code for the AI of the zombies.

My AI use a detection for don’t attacking a player already dead, for detect this I use this line of code :

		if (CurrentHealth > 0 && PlayerHealth.CurrentHealth > 0) 
			anim.SetBool ("IsIdle", false);
			nav.SetDestination (player.position);
			nav.speed = ZombieSpeed;

“CurrentHealth” is the current health of my zombie and “PlayerHealth.CurrentHealth” is the current health of the player.

So i want to detect if my zombie and my player have more than 0 health point to trigger the attack.
Unity give me this error : “NullReferenceException: Object reference not set to an instance of an object”
The problem seems to coming from the “if(…)”, but I don’t know what I’m doing wrong, all works perfectly except this.

So this piece of code is not correct ? Where is my mistake(s) ?
I looked at different websites/forum, but I’ve don’t found a solution.

first you havent set reference anim in the script.
is your CurrentHealth is static?