What's wrong with my code? (Collision, JS)

In my FPS you have like a plataform and if you fall from it you're supposed to die when you hit the ground, so i made a box with a collider (removed mesh renderer and added "FallDead" tag) then i called this is my CharacterLife script (inside the Player GameObject):

 function OnCollisionEnter (collision : Collision){
     if(collision.gameObject.tag == "Enemy"){
         totalHealth = totalHealth - 10;
     }
     if(collision.gameObject.tag == "FallDead"){
         totalHealth = 0;
     }
 }

When i collide with my enemy (with the tag "Enemy") my player loses 10 health, and this part works perfect, but the second one, checking if the player hits the ground, is not working.

The player has no Rigidbody but the Ground game object has (with freezed rotation and position, and no gravity) and I CAN'T add a rigidbody to my player, i tried that once and the physics went crazy everything started flying and stuff like that...

Also I apologize about my english i'm not american.