I can't set AudioSource pitch. It said "Value cannot be null"

here's the script.

         void Start()
         {
             this.gameObject.GetComponent<AudioSource>().pitch = 1.4f;
         }

and I got this error after I play it.

 ArgumentNullException: Value cannot be null.
 Parameter name: source

edit: I tried change the pitch from another script and then it works. But still want to know why can't i change the pitch from this script.