i am using a code to make my cube walk with character controller ,but when i play the cube spins.

 #pragma strict
 
 var speed : float = 6.0;
 var jumpSpeed : float = 8.0;
 var gravity : float = 20.0;
 private var moveDirection : Vector3 = Vector3.zero;
 
 
 function Update ()
 
 {
                 var controller : CharacterController = GetComponent.<CharacterController>();
                 if (controller.isGrounded) {
             
             moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                     Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
             if (Input.GetButton ("Jump")) {
                 moveDirection.y = jumpSpeed;
             }
         }
 
          
         moveDirection.y -= gravity * Time.deltaTime;
         
         
         controller.Move(moveDirection * Time.deltaTime);
     
     
 }