Unable to make LocalLB.VirtualServer get_default_pool_name work

All,

New to icontrol and php - pardon my knowledge. Writing a php to get VS , Pool Name and Status of VS. Im able to pull the status of VS and VS Name. However, the pool name would not load and comes up blank. I also see the error - Trying to get property of non-object in /var/www/html/learn/test.php

Any advise?

Code -

$vs_list=$client->get_list();
   sort($vs_list);
   $vsstatus=$client->get_object_status($vs_list);
   $poollist=$client->get_default_pool_name($vs_list);

     .............................

      foreach ($vs_list as $index=>$vs)
      {
         $pool_name=$poollist[$index]->pool_name;
         $availability_status=$vsstatus[$index]->availability_status;
         $img = getgif($availability_status);
        echo "";
        echo "$vs";
        echo "$pool_name";
        echo "";
        echo "";
      }

-Sridhar