php soap响应为null，但__last_response不是

我使用nusoap作为服务文件这里是我的服务.php

require_once "lib/nusoap.php";
function getProd($category) {
    if ($category['category'] == "books") {
        return join(",", array(
            "The WordPress Anthology",
            "PHP Master: Write Cutting Edge Code",
            "Build Your Own Website the Right Way")
        );
    } else {
        return "No products listed under that category: ".$category['category'];
    }
}
$server = new soap_server();
$server->configureWSDL("productlist", "urn:productlist");
$server->register("getProd",
    array("category" => "xsd:string"),
    array("return" => "xsd:string"),
    "urn:productlist",
    "urn:productlist#getProd",
    "rpc",
    "encoded",
    "Get a listing of products by category"
);
$post = file_get_contents('php://input');
$server->service($post);

生成的WSDL文件url是

https://doktormobil.ru/cms/soap/service.php?wsdl

我的客户.php是

$client = new SoapClient("https://doktormobil.ru/cms/soap/service.php?wsdl", array('trace' => 1));
$params = array("category" => "books");
$response = $client->getProd($params);
var_dump($client);
var_dump($response);

响应为NULL，但在$client->__last_Response中，我从Service得到了正确的答案。

问题出在哪里？

谢谢。