Problema com Client SOAP

Boa tarde,

estou tendo o seguinte erro ao tentar comsumir um serviço Ws:

An error occurred Application error Exception information:Message: Value cannot be null. Parameter name: Internal Error: The instance of the MessageContract cannot be null in http://schemas.navitaire.com/WebServices/ISessionManager/Logon.

Stack trace:#0 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Soap\Client.php(1121): SoapClient->__soapCall('Logon', Array, NULL, NULL, Array)#1 C:\Program Files\Zend\Apache2\htdocs\voengine\application\webservices\wsclient\company\WSCompanyGW.php(40): Zend_Soap_Client->__call('Logon', Array)#2 C:\Program Files\Zend\Apache2\htdocs\voengine\application\webservices\wsclient\company\WSCompanyGW.php(40): Zend_Soap_Client->Logon(Array)#3 C:\Program Files\Zend\Apache2\htdocs\voengine\application\controllers\IndexController.php(20): WSClient_Company_WSCompanyGW->newLogon()#4 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Controller\Action.php(516): IndexController->indexAction()#5 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Controller\Dispatcher\Standard.php(308): Zend_Controller_Action->dispatch('indexAction')#6 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Controller\Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))#7 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Application\Bootstrap\Bootstrap.php(97): Zend_Controller_Front->dispatch()#8 C:\Program Files\Zend\ZendServer\share\ZendFramework\library\Zend\Application.php(366): Zend_Application_Bootstrap_Bootstrap->run()#9 C:\Program Files\Zend\Apache2\htdocs\voengine\public\index.php(26): Zend_Application->run()#10 {main} Request Parameters:array ( 'controller' => 'index', 'action' => 'index', 'module' => 'default',)

Agradeço desde já qualquer dica.

Att

Sani Naimayer