The null value automatically changes to 0

I'm doing a vehicle registration, I need it to register that whenever a vehicle is registered to zero instead of the value null, I tried by default, but it does not record the information.

I'm using SQL Server along with C # in visual studio.

Controller class:

public ActionResult Adiciona(VeiculoModel viewModel)
{
  if (ModelState.IsValid)
  {
    Veiculo Veiculo = viewModel.CriaVeiculo();
    dao.Adiciona(Veiculo);
    //return View();
    return RedirectToAction("Form");
  }
  else
  {
    return View("Index", viewModel);
  }
}

DAO Class:

public void Adiciona(Veiculo post)
{
  ITransaction tx = session.BeginTransaction();
  session.Save(post);
  tx.Commit();
}