ListBox.SelectedValue returning null?

I have a listbox in an ASP.NET web application that stores a list of names.  These names are bound to a database that stores the names and their associated IDs, and is referenced via LINQ to SQL.  Whenever I attempt to delete a name from the list, I encounter a FormatException because ListBox1.SelectedValue is "" (it should be an ID that is the value of a name).  Any ideas about why this is happening?  Thanks in advance!
protected void checkOutButton_Click(object sender, EventArgs e) { DataClasses1DataContext db = new DataClasses1DataContext(); var selectedID = int.Parse(ListBox1.SelectedValue); tblNorthAttendee toDelete = db.tblNorthAttendees.Single(a => a.id == selectedID); db.tblNorthAttendees.DeleteOnSubmit( toDelete ); db.SubmitChanges(); ListBox1.DataBind(); }