在NSUserDefaults中保存类的奇怪问题

Question

我有下一堂课：

class UnreadMessages: NSObject {
let publicID: String
let messagesCount: Int

init(publicID: String, messagesCount: Int) {
    self.publicID = publicID
    self.messagesCount = messagesCount
}

required init(coder aDecoder: NSCoder) {
    self.publicID = aDecoder.decodeObjectForKey("publicID") as! String
    self.messagesCount = aDecoder.decodeObjectForKey("messagesCount") as! Int
}

func encodeWithCoder(aCoder: NSCoder) {
    aCoder.encodeObject(publicID, forKey: "publicID")
    aCoder.encodeObject(messagesCount, forKey: "messagesCount")
}
}

我将它保存在NSUserDefault中：

var chatList = [UnreadMessages]()
NSUserDefaults.standardUserDefaults().setObject(NSKeyedArchiver.archivedDataWithRootObject(chatList), forKey: "ChatList")

然后我尝试将这些值设为：

if let data = NSUserDefaults.standardUserDefaults().objectForKey("ChatList") as? NSData {
        let blog = NSKeyedUnarchiver.unarchiveObjectWithData(data)
        print(blog)
    }

但是当我尝试打印blog.publicID时，它会给我下一个错误：

error: <EXPR>:2:10: error: 'UnreadMessages' is ambiguous for type lookup in this context blog as! UnreadMessages

我该如何解决？