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);
	}
}

var options is of type ShowOptions, but since it’s either a struct or a class, you need to create one with new.