[HELP] Setting the parent of a transform which resides in a prefab is disabled

Hi!

Im trying to instantiate a prefab from an array as a game object as a child of EnemySpawner GameObject.

The weird thing is, the first enemy spawns properly and is properly placed as a child. But the following one spawns not as a child and I get the error "Setting the parent of a transform which resides in a prefab is disabled"

 using UnityEngine;
 using System.Collections;
 
 public class EnemySpawner : MonoBehaviour {
 
     public GameObject[] enemies;
     
     public GameObject enemy;
     
     
     void Start(){
             Spawn();
     }
     
     public void Spawn(){
             Debug.Log ("Spawning new enemy!");
             enemy = Instantiate (enemies[Random.Range(0,4)],transform.position,Quaternion.identity) as GameObject;    
             enemy.transform.parent = transform;
 
     }
     
 
 }

And here is my enemy script if you need it.

 using UnityEngine;
 using System.Collections;
 
 public class EnemyBehaviour : MonoBehaviour {
 
     public EnemySpawner spawnScript;
     public float health;
     public GameObject[] enemy;
 
     
     void OnTriggerEnter2D(Collider2D collider){
         Projectile bullet = collider.gameObject.GetComponent<Projectile>();
         if(bullet){
             health -= bullet.GetDamage();
             bullet.Hit();
             if (health <= 0){    
                 Destroy(gameObject);
                 spawnScript.Spawn();
                 Debug.Log ("Enemy Destroyed");    
                 
             }
         }
     }
 }
 

What am I missing to put the following prefabs into the right parent?