小程序支付，创建订单的时候，提交的如下的Json字符串，没看出哪里有问题啊，返回400错误

远程服务器返回错误: (400) 错误的请求。

请指教，谢谢

{

"mchid": "123456789",

"out_trade_no": "20230817193938948",

"appid": "wxdbd61123456789",

"description": "orderId:20230817193938948",

"notify_url": "https://www.xxx.com//pay/Wx3_notify_url",

"amount": {

"total": 21000,

"currency": "CNY"

},

"payer": {

"openid": "og3RI5MNYLlaGb123456789"

}

}