Why is resultCallback coming up with a NullReferenceException in Javascript?

So I am trying to use Unity Ads with javascript, but each time I try to click on the button it come up with:

"NullReferenceException: Object reference not set to an instance of an object AdButton.OnMouseDown () (at Assets/AdButton.js:22) UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32) "

Here is the code that I am using, any help would be greatly appreciated!

 #pragma strict
 
 var ad : UnityEngine.Advertisements.Advertisement;
 
 function HandleShowResult (result: UnityEngine.Advertisements.ShowResult) {
     switch (result) {
         case UnityEngine.Advertisements.ShowResult.Finished :
             Debug.Log("The ad was successfully shown.");
             break;
         case UnityEngine.Advertisements.ShowResult.Skipped :
             Debug.Log("The ad was skipped");
             break;
         case UnityEngine.Advertisements.ShowResult.Failed :
             Debug.LogError("The ad failed to be shown.");
             break;
         } 
     }
 
 function OnMouseDown () {
     if (ad.IsReady("rewardedVideo")) {
         var options : UnityEngine.Advertisements.ShowOptions;
         options.resultCallback = HandleShowResult;
         ad.Show("rewardedVideo", options);
     }
 }