NullReferenceException question.

 void OnTriggerEnter(Collider collider)
 {
     if (collider.gameObject.tag == "bullet")
     {
         manager.SendMessage("Box",collider,SendMessageOptions.DontRequireReceiver);
     }
 }

Console says collider is a null reference. How come? All objects' colliders and triggers are enabled. How could an active collider be null? Thanks in advance.

Edit : My manager variable:

     GameObject manager;
 
     // Use this for initialization
     void Start () {
         manager = GameObject.Find("GameManager");
     }