Cannot assign to property: 'detailDic' is a 'let' constant

I have issue with my code, it tells me "Cannot assign to property: 'detailDic' is a 'let' constant" and my build failed

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "detail" {
            if let indexPath = self.tableView.indexPathForSelectedRow {
                let dic = cities[indexPath.row]
                let detailVC = segue.destination as! DetailViewController
                detailVC.detailDic = dic
                
                
            }
        }
    }

I've tried this and got this error "Result of operator '==' is unused" and it runs but it will be crash

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "detail" {
            if let indexPath = self.tableView.indexPathForSelectedRow {
                let dic = cities[indexPath.row]
                let detailVC = segue.destination as! DetailViewController
                detailVC.detailDic == dic
                
                
            }
        }
    }

i found my problem

i used let detailDic = [String:String]()

that most be var detailDic = [String:String]()