NuSOAP不发送凭据

I'm trying to connect to a SOAP server secured with HTTP Basic Authentication and SSL. I tried using PHP's SOAP library, but then gave up and used NuSAOP. I set my credentials:

$soap = new nusoap_client('https://ws.merchanttest.firstdataglobalgateway.com/fdggwsapi/services/order.wsdl', 'wsdl');
$soap->setCredentials('WS'.STORE_NUMBER.'._.1',
 PASSWORD,
 'certificate',
 array(
     'sslcertfile' => 'first_data/WS'.STORE_NUMBER.'._.1.pem',
     'sslkeyfile' => 'first_data/WS'.STORE_NUMBER.'._.1.key',
     'passphrase' => KEY_PASSPHRASE
 )
);
$proxy = $soap->getProxy();
$proxy->someRequest($data);

However, the credentials don't appear to send, for here's the header of the request:

POST /fdggwsapi/services HTTP/1.0
Host: ws.merchanttest.firstdataglobalgateway.com:443
User-Agent: NuSOAP/0.9.5 (1.123)
Content-Type: text/xml; charset=ISO-8859-1
SOAPAction: ""
Content-Length: 489

I get back a 401 Not Authorized response. How can I get the credentials to send?