         // Extract the requested formats to return.
         for each (p in names)
         {
+            if (!(p in description))
+                continue;
+                
             category = description[p].category;
             
             if (category == Category.CONTAINER && containerFormat)

        
        for (var p:String in attributes)
        {
            var category:String = description[p].category;
            
            if (category == Category.CONTAINER)

        {
            for each (p in names)
            {
                if (!(p in description))
                    continue;
                    
                category = description[p].category;

                if (category == Category.CONTAINER)
