code not appearing in HEAD section

I am trying to add the following script between the HEAD tag of AMP pages (since I manually add adverts in my functions.php file instead of using the plugin advert feature.)

<script async custom-element=”amp-ad” src=”https://cdn.ampproject.org/v0/amp-ad-0.1.js”></script&gt;

I used the plugin’s Advanced Settings to add this script in the head section, but it doesn’t appear in the resulting HTML page’s source code? Any idea why?

Thanks