Why is __getLastRequest() returning null?

<?php
$WDSL = 'http://localhost/QB_build_20130416/QuickBooks/TEST.wsdl';

$client = new SoapClient( $WDSL, array('trace' => 1)); 
var_dump($client->__getFunctions());
var_dump($client->__getLastRequest());
?>

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array (size=8)
  0 => string 'serverVersionResponse serverVersion(serverVersion $parameters)' (length=62)
  1 => string 'clientVersionResponse clientVersion(clientVersion $parameters)' (length=62)
  2 => string 'authenticateResponse authenticate(authenticate $parameters)' (length=59)
  3 => string 'sendRequestXMLResponse sendRequestXML(sendRequestXML $parameters)' (length=65)
  4 => string 'receiveResponseXMLResponse receiveResponseXML(receiveResponseXML $parameters)' (length=77)
  5 => string 'connectionErrorResponse connectionError(connectionError $parameters)' (length=68)
  6 => string 'getLastErrorResponse getLastError(getLastError $parameters)' (length=59)
  7 => string 'closeConnectionResponse closeConnection(closeConnection $parameters)' (length=68)
null

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