add a list to the database

Well im doing a application where i have a model like this

public class Saldo
{
    public Saldo()
    {
        Expenses = new List<Expese>();
        Incomes = new List<Income>();
    }

    public int SaldoId { get; set; }
    public List<Expense> Despesas { get; set; }
    public List<Income> Rendimentos { get; set; }
    public string ApplicationUserId { get; set; }
}

what i want to do is add a single expense to the list but it is not working, when i output on the console the count it is always null in the controller im doing this:

 public ActionResult Create([Bind(Include = "ExpenseId,TipoDespesaId,DespesaDescricao,DespesaValor,TipoPagamentoId,Data,Comentario")] Expense expense)
    {
        var userId = User.Identity.GetUserId();
        if (ModelState.IsValid)
        {

            var balance = db.Balance.Where(d => d.ApplicationUserId == userId).FirstOrDefault();


            expense.ApplicationUserId = userId;

            if (balance == null)
            {
                Balance s = new Balance();
                s.Expense.Add(expense);
                s.ApplicationUserId = userId;
                db.Balance.Add(s);             
            }
            else
            {
                Balance.Expense.Add(expense);
            }

            db.Expense.Add(expense);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

The expense comes from the form and is passed to the Action

I created a new instance of Balance it worked adding the ApplicationUserId but the expense on the list didnt work, can someone explain me why it happen?

Ps: Sorry for my bad english