GUIText update error

GUITextUpdate.js

private var numPoints= 0;

 function Start()
 {
     UpdateDisplay();
 }
 
 function UpdateDisplay()
 {
     guiText.text = "Points: "+numPoints;
 }
 
 function AddPoints()
 {
     numPoints++;
     UpdateDisplay();
 }

DetectCollision.js

var gu: GUITextUpdate;

   function OnCollisionEnter(theCollision : Collision){

  if(theCollision.gameObject.name == "Wicket"){

       Debug.Log("Hit the cube");

       gu.AddPoints();

  }

 }

giving this error

NullReferenceException: Object reference not set to an instance of an object DetectCollission.OnCollisionEnter (UnityEngine.Collision theCollision) (at Assets/Script/DetectCollission.js:6)