collapse/expand on mouseclick

i would like to expand or collapse the group by clicking on the group name.
my code crashes at e.Item.Group because Group is null.
why is group null when i click on the group name?

        private void Operation_lv_ItemMouseClick(object sender, ListViewItemEventArgs e) {
            if (e.Item.Group != null) {
                e.Item.Group.Expanded = !e.Item.Group.Expanded;
            }
        }

many thanks