Script blocking box for the Header won’t ouput scripts

Hi,

I just added this script on the Script Blocking box / Header area:

start testing

<script type="text/javascript">(function(d, s, id) {
  var js, fjs = d.getElementsByTagName(s)[0];
  if (d.getElementById(id)) return;
  js = d.createElement(s); js.id = id;
  js.src = "//connect.facebook.net/en_US/sdk.js#xfbml=1&version=v2.4";
  fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>

end testing

I can see the start testing and end testing bits (plain text) but the script itself won’t appear!

What can be wrong? Thanks,