保存到钥匙串时返回状态码-50

Question

根据Apple文档学习如何在Swift中使用钥匙串，当我尝试保存时，总是会得到状态代码-50。

有人可以帮忙吗？

这是我的代码：

import UIKit
import Security

let login = "samplelogin"
let password = "samplepassword"

let data = password.data(using: String.Encoding.utf8)!

let query:[String: Any] = [kSecClass as String: kSecClassGenericPassword,
                          kSecAttrAccount as String: login,
                          kSecValueData as String: data]

let status = SecItemAdd(query as CFDictionary, nil)