Cannot update List1: it has no PK

Question

I'm trying to update a record in a database, the changes are made on the list that I create for the page, but when I try to update the database itself, I get an error that states

Cannot update List1: it has no PK

The application breaks on the last line of code. My table has a primary key. It should be Tno in Students.

private void ChangeMajor_Button_Click(object sender, RoutedEventArgs e)
{
    var query = Roster_Students.Where(s => s.Tno == Tno_TextBox.Text);
    foreach (var student in query)
        student.Major = ChangeMajor_TextBox.Text;

    App.DBConnection.Update(this.Roster_Students);        
}

Answer 1

Modifying the code a little seemed to resolve the issue. Here is the new code:

private void ChangeMajor_Button_Click(object sender, RoutedEventArgs e)
{
    var query = Roster_Students.Where(s => s.Tno == Tno_TextBox.Text);
    foreach (var student in query) {
        student.Major = ChangeMajor_TextBox.Text;
        App.DBConnection.Update(student);
    }
}