Why do i get NullReferenceException here?

 List<Collider> trig;
 // Use this for initialization
 void Start()
 {

 }
 private void OnTriggerStay(Collider other)
 {
     if (other.gameObject.tag == "Zombie")
     {

         if (!trig.Contains(other))
         {
             trig.Add(other);
         }
         for (int i = 0; i < trig.Count; i++)
         {
             Vector3 dir = trig[i].gameObject.transform.position.normalized*-1;
             transform.Translate(dir * 0.5f * Time.deltaTime);
         }


     }

     // Update is called once per frame

 }
 private void OnTriggerExit(Collider other)
 {
     if (trig.Contains(other)) {
         trig.Remove(other);
     }
 }