json not always defined in Drupal.mnc.requestUnreadCounter

For some reason, on some page requests, some users are having an issue where "json" object in Drupal.mnc.requestUnreadCounter is sometimes null.

See these screenshots:

http://i.imgur.com/mm5H7ZE.png

http://i.imgur.com/0qGYbBh.png

Could put an if(json) check to avoid errors, but any ideas on why this might be happening? I'm running Apache behind Varnish and CloudFlare, and not sure if those "layers" might be contributing...

Relevant code chunk:

  var data = $.ajax({
      type: 'POST',
      url: Drupal.settings.basePath + 'mnc/ajax-unread-count',
      dataType: 'json',
      async: false
    });
  var json = jQuery.parseJSON(data.responseText);