PHP Soap Object reference not set to an instance of an object

I'm trying to login to a URL and the following error appears:

  

Object reference not set to an instance of an object.

Follow the code:

<?php
$clientSoap = new SoapClient( "http://www1.snd.com.br/Plugins/Atma.Plugin.Misc.ServiceAPI/Controllers/ServiceAPIEndpoint.svc?singleWsdl" );
$params = array('Login' => array(
                    'Usuario'   => 'usuario',
                    'Senha'     => 'senha'));


$result = $clientSoap->__soapCall('Login', $params);

print_r($result);      

?>

I have already changed and tested in many ways and always gives the same error.