AOG Dialogflow实现-PHP Webhook响应为null

Question

我用PHP实现了实现Webhook。我在实现配置页面中添加了URL。但是，当我试图通过Google上的“操作”表达我的意图时，这是我的错误：

**MalformedResponse Failed to parse Dialogflow response into AppResponse because of empty speech response.**

查看日志：

Webhook call failed. Error: Failed to parse webhook JSON response: null

我的php代码：

$return_defaults = 
    array(
        "fulfillmentText" => "respons of it",
        "fulfillmentMessages" => array(
            array(
                "text" => array(
                "text reponse"
            ),
        )
    ),
    "payload" => array(
        "google" => array (
            "expectUserResponse" => true,
            "richResponse" => array(
                "items" => array(
                    array(
                        "simpleResponse" => array(
                            "textToSpeech" => "bye bye calamba"
                        ),
                    ),
                ),
            ),
        ),
    ),
);

$ReturnValue = json_encode($return_defaults);
echo $ReturnValue;