我得到系统.ObjectDisposedException，似乎找不到什么问题

在第一个add方法中，我得到了异常:ObjectContext实例已被处理，不能再用于需要连接的操作。

我知道我需要在using()括号中，它是，事务是公共虚拟的

  public void confirmPayments(List<int> Payments)
    {
        using (var db = new BankContext())
        {
        try
        {
                foreach (int a in Payments)
                {
                    foreach (Payment payment in db.Payment)
                    {
                        if (a == payment.Id)
                        {
                            Transaction senderTransaction = new Transaction
                            {
                                Amount = payment.Amount,
                                CreationDate = payment.CreationDate,
                                DueDate = payment.DueDate,
                                Receiver = payment.Receiver,
                                Sender = payment.Sender,
                                KIDMessage = payment.KIDMessage
                            };
                            Transaction receiverTranscation = new Transaction
                            {
                                Amount = payment.Amount,
                                CreationDate = payment.CreationDate,
                                DueDate = payment.DueDate,
                                Receiver = payment.Receiver,
                                Sender = payment.Sender,
                                KIDMessage = payment.KIDMessage
                            };
                            Account senderAccount = findAccount(senderTransaction.Sender);
                            Account receiverAccount = findAccount(receiverTranscation.Receiver);
                            senderAccount.Transaction.Add(senderTransaction);
                            senderAccount.Payment.Remove(payment);
                            senderAccount.Balance = senderAccount.Balance - senderTransaction.Amount;
                            receiverAccount.Transaction.Add(receiverTranscation);
                            receiverAccount.Balance = receiverAccount.Balance + receiverTranscation.Amount;
                        }
                    }
                }
                db.SaveChanges();
        }
        catch (Exception error)
            {
                System.Diagnostics.Debug.WriteLine("FAILED TO CONFIRM PAYMENT: 'n" + error.Message + "'n");
            }
        }
    }

我想添加到的表/类(codefirst):

    public class Account
{
    [Key]
    public int Id { get; set; }
    public double Balance { get; set; }
    public string AccountNumber { get; set; }
    public string AccountType { get; set; }
    public string CreationDate { get; set; }
    public string ClosingDate { get; set; }
    public virtual List<Transaction> Transaction { get; set; }
    public virtual List<Payment> Payment { get; set; }
}