Trying to allow nulls but... "Nullable object must have a value"

Question

I am trying to allow nulls in my drop down list, in my database table I have set allow nulls for that specific field which is int, but when I run the code I get error saying "Nullable object must have a value", I think problem may be in ModelState.

Controller

[HttpPost]
    public ActionResult Edit(Student student)
    {
        if (ModelState.IsValid)
        {
            db.Entry(student).State = EntityState.Modified;
            db.SaveChanges();
            Loan w = new Loan()
            {
                StudentID = student.StudentID,
                ISBN = student.ISBN.Value,
            };
            db.Loans.Add(w);
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        ViewBag.ISBN1 = new SelectList(db.Books, "ISBN", "Titulli", student.ISBN);
        return View(student);
    }

Answer 1

You are getting this error when trying to get value of nullable object, which do not have value. If Loan.ISBN property is not nullable then you should provide default value for that property

ISBN = student.ISBN.HasValue ? student.ISBN.Value : defaultValue
// or ISBN = student.ISBN ?? defaultValue
// or ISBN = student.ISBN.GetValueOrDefault()

If Loan.ISBN property is nullable, then simply assign student.ISBN without accessing Value of nullable type

ISBN = student.ISBN