 
     public function checkViewability():Object
     {
+      if (!_initialized)
+        raiseError("ExternalInterface unavailable");
+
       return performCheck();
     }
 

    private static const IMPRESSION_DELAY:Number = 250;
    private static const VIEWABLE_AREA_THRESHOLD:Number = 50;

    private var _id:String;
    private var _viewabilityCheck:OVVCheck;
    private var _impressionTimer:Timer;

    {
      _id = id;

      if (!externalInterfaceIsAvailable())
      {
        raiseError("ExternalInterface unavailable");
        return;
      }

      _initialized = true;
      _viewabilityCheck = new OVVCheck(_id);

      _intervalsInView = 0;
