Cannot pass GameObject found by raycast

I use raycast, found the object (debug.log shows it exist) however when i try to pass it to another object, got NullReferenceError Object reference not set to an instance of an Object C#

 private void CheckPick() {
             Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
             RaycastHit hit;
             if (iInput.touchCount > 0 && iInput.GetTouch (0).phase == TouchPhase.Began) {
                 if (Physics.Raycast (ray, out hit, Mathf.Infinity)) {
     
                     if (hit.collider.gameObject.tag == "EnemyMissile"){
                         Debug.DrawLine (ray.origin, hit.point);
                         GameObject em = hit.collider.gameObject;
                         PlayerMissile m;
                         m = Instantiate(missilePrefab, em.transform.position, Quaternion.identity) as PlayerMissile;
                         Debug.Log ("missile is " + em); // this shows it found GameObject. works fine.
                         m.enemyMissile = em; // got NullReferenceError here. error when use this code

enemyMissile is public variable, type GameObject,in another script