swift – 尝试追加/替换数组元素时“对成员映射的模糊引用”

下面的代码为地图调用生成“对成员地图的模糊引用”错误.

谁知道为什么？

func saveUser(user: User) {
    var userDicts = masterDict["users"] as! [[String:AnyObject]]
    let newDict = user.getDict()

    // Replace matching element with <newDict>
    let replaced = false
    masterDict["users"] = userDicts.map {
        if ($0["id"] as String! == user.getId()) {
            replaced = true
            return newDict
        } else {
            return $0 as [String:AnyObject]
        }
    }

    // If no match found,means must add new user
    if (!replaced) {
        userDicts.append(newDict)
    }
}