Invalid Key or Ref - Please help

I am using the PHP Tool Kit and I am trying to add items to an order...

This is my code:

$SalesOrderRecord  = new nsComplexObject('SalesOrder');

$item1     = new nsComplexObject('SalesOrderItem');
$item2     = new nsComplexObject('SalesOrderItem');

$item1     = array( 'item' => new nsComplexObject('RecordRef', array('internalId' => '6529')), 'quantity'  => '1');
$item2     = array( 'item' => new nsComplexObject('RecordRef', array('internalId' => '6528')), 'quantity'  => '1');

$salesOrderFields  = array(
    'entity'        => new nsRecordRef(array('externalId' => $customerId)),
    'itemList'   => array('item' => array($item1, $item2)));

$SalesOrderRecord  -> setFields($salesOrderFields);
$addResponse   = $myNSclient->add($SalesOrderRecord);

This is the response that I get.