Adwords Remarketing Javascript (Conversion_Async. Js) Is Not Loading

Per the Google Asynchronous AdWords Remarketing tag instructions I add the following snippet inside the head section of my page:

<script type="text/javascript" src="" charset="utf-8"></script>

But I'm unable to call the google_trackConversion function after page load:

<script type="text/javascript">
/* <![CDATA[ */
window.google_trackConversion({
  google_conversion_id: 123456789, 
  google_custom_params: {
    parameter1: 'abc123', 
    parameter2: 29.99
  },
  google_remarketing_only: true
});
//]]>
</script>

However if copy/paste the code from conversion_async.js to the head section it works just fine, indicating the conversion_async.js file is not loading from the googleadservices.com server.

How can I ensure the conversion_async.js file has already been loaded before calling google_trackConversion?

(BTW, as suggested someplace else I toggled AddBlock Plus on/off but that doesn't have any effect)