Collsions problem

See i have a project im working on but cant get it to work. Cant make the bullet destroy the zombie for some reason. The script: #pragma strict

var zombiedeathSound : AudioClip;

function OnTriggerEnter (other : Collider) { if(other.tag == "Bullet") { audio.PlayOneShot (zombiedeathSound); yield WaitForSeconds (0.4); Destroy(gameObject);

 }

} . Here i have some pictures of my project so you can understand all of it: