c# Object reference not set to an instance of an object

 using UnityEngine;
 using System.Collections;
 
 public class enemy_movement : MonoBehaviour {
 
         int i=0;
         int num=-1;
         int temp;
         public Transform[] patrolpoints;
         public Transform planetposition;
     public int movespeed;
     private int currentposition;
     public int size;
     GameObject[] enemy;
     GameObject PreFab;
     
     // Use this for initialization
     void Start () {
     
     
     PreFab=(GameObject)Instantiate(Resources.Load("enemy"));
     //currentposition=0;
     }
     
     
     
     
     // Update is called once per frame
     void Update () {
     
     
     
     if(num<2)
     {
     num=num+1;
     temp=random_value();
     
     //this is the error line
 
 
     enemy[num] = (GameObject)GameObject.Instantiate(PreFab, patrolpoints[temp].position, Quaternion.identity);
     
     
     
     }
         for(i=0;i<=num;i++)
         enemy[i].transform.position=Vector3.MoveTowards(enemy[i].transform.position, planetposition.position , movespeed*Time.deltaTime);
     }
     
     
     int random_value()
     {
     int num = Random.Range(0, patrolpoints.Length);
 return num;
     }
 }

i'm new to c#,building my first game on unity.

the error that i get is NullReferenceException: Object reference not set to an instance of an object (wrapper stelemref) object:stelemref (object,intptr,object) enemy_movement.Update () (at Assets/scripts/enemy_movement.cs:50)

this code works for a single enemy ,but not for multiple that are created using an array. any way to correct this or any better way to do this.

thanks.