NullReferenceException: Object reference not set to an instance of an object

I am trying to make an enemy movement script for my tower defence game but i keep getting this error and my enemy is not moving

NullReferenceException: Object reference not set to an instance of an object EnemyMovement.Update () (at Assets/Scripts/EnemyMovement.cs:16)

Here is my script

 public float speed = 10f;

 private Transform target;
 private int wavepointIndex = 0;

 void start() {
     target = Waypoints.points[0];
 }

 void Update() {
     Vector3 dir = target.position - transform.position;
     transform.Translate (dir.normalized * speed * Time.deltaTime, Space.World);
     if (Vector3.Distance (transform.position, target.position) <= 0.4f) {
         GetNextWaypoint();
     }
 }

 void GetNextWaypoint() {
     if (wavepointIndex >= Waypoints.points.Length -1) {
         Destroy (gameObject);
         return;
     }
     wavepointIndex++;
     target = Waypoints.points [wavepointIndex];
 }

If any one knows what the problem is your help is greatly appropriated

thanks