The selected property should only be used with ListBox components that have the MultiSelect property set to true. If you are working with a ListBox that has the MultiSelect property set to false, use the ItemIndex property to select the item. Example:

procedure TForm .Button Click(Sender: TObject);
begin
  ListBox .Items.Add(' ');
  ListBox .Items.Add('2');
{This will fail on a single selection ListBox}
//  ListBox .Selected[ ] := true;
  ListBox .ItemIndex :=  ; {This is ok}
end;