I'm building my first Facebook Connect application, and am running into an odd problem. This example code:

<script type="text/javascript" charset="utf-8">
  FB.init("{{ FACEBOOK_API_KEY }}", "{% url xd_receiver %}");
  FB.ensureInit(function() {
    var invite_dialog;
    function showInviteDialog() {
      invite_dialog = new Dialog(Dialog.DIALOG_POP).showMessage('Share with Friends', invite_friends_selector, null, null);
    }
    function hideInviteDialog() {
      invite_dialog.hide();
    }
    showInviteDialog();
  });
</script>

keeps throwing the error:

Dialog is not defined

Is Dialog available in Facebook connect/XFBML applications? Do I need to do anything special in order to use it?

Thanks in advance!