In my tower defence game my cannon wont lock onto the enemys

I have a cannon and a enemy but the cannon doesn't lock onto the enemy does anyone know why? I put a tag on the enemy prefab but Its still not working. My code is

using UnityEngine; using System.Collections;

public class Cannon : MonoBehaviour {

 public Transform target;
 public float range = 15f;

 public string NormalEnemyTag = "NormalEnemy";
 
 void Start ()
 {
     InvokeRepeating("UpdateTarget", 0f, 0.5f); 
 }

 void UpdateTarget ()
 {
     GameObject[] NormalEnemies = GameObject.FindGameObjectsWithTag(NormalEnemyTag);
     float shortestDistance = Mathf.Infinity;
     GameObject nearestNormalEnemy = null;

     foreach (GameObject NormalEnemy in NormalEnemies)
     {
         float distanceToNormalEnemy = Vector3.Distance(transform.position, NormalEnemy.transform.position);
         if (distanceToNormalEnemy < shortestDistance)
         {
             shortestDistance = distanceToNormalEnemy;
             nearestNormalEnemy = NormalEnemy;
         }
     }

     if (nearestNormalEnemy != null && shortestDistance <= range)
     {
         target = nearestNormalEnemy.transform;
     }

 }
 



 void Update()
 {
     if (target == null) 
         return;


 }

 void OnDrawGizmosSelected ()
 {
     Gizmos.color = Color.red;
     Gizmos.DrawWireSphere(transform.position, range);
 }

}