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

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

        // 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)
