Binder, setBean error

I have the following Code

private ComboBox<String> cbYear 		 = new ComboBox<>("Jahr *");

private Button refreshBtn = new Button("Lade Daten...");
private Binder<DateSelection> binder = new Binder<>(DateSelection.class);
private DateSelection dateSelection = new DateSelection();

public MainView() {
	//.. some Code ...

	binder.forField(cbYear).bind(DateSelection::getYear, DateSelection::setYear);


It caueses an error if I don’t select something in the combobox. But I want the field to be not required. How can I handle this?