I'm trying to make a SOAP call to a web service, but I'm getting this error:

"Server was unable to process request. ---> Object reference not set to an instance of an object."

Any idea what would cause this?

Here's the code:

PHP Code:
try {    
    
$params = array(xxx);

    
$client = new SoapClient("http://xxx.com/yyy.wsdl");

      
$response $client->PostFile($params);

      echo 
$response;
    } catch (
Exception $e) {
           
printf("Message = %s\n",$e->__toString());
    }