How can i fix this error?

I am making a game in unity and cant get rid of this error :

Assets/Scripts/BallControl.js(14,22): BCE0034: Expressions in statements must only be executed for their side-effects.

Does anyone know how to get rid of it?

Here is my script:

 #pragma strict
 
 var rotationSpeed = 100;
 var jumpHeight = 8;
 
 var Hit01 : AudioClip;
 var Hit02 : AudioClip;
 var Hit03 : AudioClip;
 
 var distToGround : float;
 
 function Start () {
     // Getting the distance from the center to the ground.
     distToGround + collider.bounds.extents.y;
 }
 
 function Update () 
 {
     //Handle ball rotation.
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     
     if (Input.GetKeyDown(KeyCode.W) && IsGrounded ())
     {
         rigidbody.velocity.y = jumpHeight;
     }
 }
 
 function IsGrounded () : boolean { //Check if we are on the ground. Return true if we are else return null.
     return Physics.Raycast(transform.position, -Vector3.up, distToGround + 0.1);
 }
 
 function OnCollisionEnter () {
     var theHit = Random.Range(0, 3);
     if (theHit == 0)
     {
     audio.clip = Hit01;
     }
     else if (theHit == 1)
     {
     audio.clip = Hit02;
     }
     else {
         audio.clip = Hit03;
     }
     audio.pitch = Random.Range (0.9,1.1);
     audio.Play();
 }