Checking Next Element

I'm trying to run some code when a H2 element is clicked on an unordered list below it. The problem is, the code runs whether the h2 has a ul below it or not.

h[x].onclick = function(){
var ul = this.nextSibling;

while (ul.nodeType != 1){
ul = ul.nextSibling;
}

So, if I have something like this:

<h2>Subtitle</h2>
<h2>Subtitle 2</h2>
<ul><li>List</li></ul>

Clicking "Subtitle" runs the function on "Subtitle 2". I don't want the function to run unless the next element is an ul. How can I verify that the next element is an unordered list and if not, don't run anything?