Inconsistent script results/error

Hi there,

I'm working on a simple health script for my game. It should be simple- if the object in which healthscript.cs is attached gets hit by a player bullet, it takes damage. When its HP = 0, it should return to the pool (a.k.a. be destroyed).

It seems like the script works every 2 out of 10 times: The bullet hits the enemy, which is destroyed and returned to the pool. All the other times, the enemy absorbs bullets like a sponge and the console throws this error every time the shot hits:

NullReferenceException: Object reference not set to an instance of an object healthscript.OnTriggerEnter2D (UnityEngine.Collider2D otherCollider) (at Assets/Scripts/healthscript.cs:22)

Here is my health script, where the magic (and errors) happen:

 using UnityEngine;
 using System.Collections;
 
 public class healthscript : MonoBehaviour
 {
 
         public int health = 1;
         public bool isEnemy = true;
 
         public void Damage (int damageCount)
         {
                 health -= damageCount;
                 if (health <= 0) {
                         GameObject obj = explodepool.current.GetPooledObject ();
                         obj.transform.position = transform.position;
                         obj.transform.rotation = transform.rotation;
                         obj.SetActive (true);
 
                         gameObject.SetActive (false);
                 }
         }
 
         void OnTriggerEnter2D (Collider2D otherCollider)
         {
                 playerbulletscript shot = otherCollider.gameObject.GetComponent<playerbulletscript> ();
                 if (shot != null) {
                         if (shot.isEnemyShot != isEnemy) {
                                 Damage (shot.damage);
                         }
                 }
         }
 }