Codable Bug?

codablePlayground.swift

//: Playground - noun: a place where people can play

import UIKit

struct User: Codable  {
    var id: String?
    var type: String?
    var url: String?
    var mobileNumber: String?
    
    var email: String?
    var firstName: String?
    var lastName: String?
    var profileImageURL: String?
    var isOnline: Bool?
    
    enum CodingKeys: String, CodingKey {
        case id
        case type
        case url
        case mobileNumber = "mobile_number"
        
        case email
        case firstName = "first_name"
        case lastName = "last_name"
        case profileImageURL = "profile_image_url"
        case isOnline = "is_online"
    }
}
struct Message: Codable {
    var id: String?
    var type: String?
    var thread_id: String?
    var threadType: String?
    var sender: User?
    var body: String?
    var media: String?
    var sentAt: Double?
    var unread: Bool?
    var status: String?
    var url: String?
    
    enum codingKeys: String, CodingKey {
        case id
        case type
        case thread_id
        case threadType = "thread_type"
        case sender
        case body
        case media
        case sentAt = "sent_at"
        case unread
        case status
        case url
    }
}
let json =
"""
{
"id": "Jvbl6LY",
"type": "sms",
"thread_id": "60LrVL7",
"thread_type": "578a",
"delay_until": null,
"sender": {
"id": "EVkdNBx",
"type": "user",
"first_name": "Jerry",
"last_name": "Ward",
"mobile_number": "123-456-7890",
"profile_image_url": "",
"is_online": false,
"email": "jerryw+demo@jypsee.com"
},
"body": "Here is a picture of our Coquille Suite.  Let me know if you would like a reservation?",
"media": "",
"sent_at": 1509133604000.167,
"unread": false,
"status": "",
"url": "https://connect-staging.jypsee.com/api/sms/threads/60LrVL7/history/Jvbl6LY/"
}
"""

let decoder = JSONDecoder()
let data = json.data(using: .utf8)!
do {
    //TODO: paginate as the user scrolls the tableView
    let message = try decoder.decode(Message.self, from: data)
    print(message.thread_id)
    print(message.threadType)
    print(message.sender?.firstName)
    print(message.sender?.lastName)
} catch {
    print(error)
}