php-PHP调用.NET写的webservice时异常，这一般是什么问题。异常错误信息如下

SoapFault exception: [soap:Server] Server was unable to process request. ---> Object reference not set to an instance of an object. in D:\www\index5.php:23 Stack trace: #0 D:\www\index5.php(23): SoapClient->__call('Pay', Array) #1 D:\www\index5.php(23): SoapClient->Pay(Array) #2 {main}

请教各位大牛，这什么问题