ui.item.prev() and ui.item.next() function differently than in previous version of Sortable

When I move items to the very top of a list, this following block of code does not function like it used to. Previously, moving an item to the top of the list (in my example), would return nextId = 234, currentId = 123, prevId = undefined. Now nextId = '' (empty string), currentId = 123, prevId = undefined. The nextId equaling empty string is the problem. None of my code changed, except I upgraded to UI 1.6 RC5 and jQuery 1.3

obj.sortable({ axis: "y", cursor: "move",
	update: function(e, ui)
        {
            var prevId = ui.item.prev().attr("id");
            var currentId = ui.item.attr("id");
            var nextId = ui.item.next().attr("id");
        }
});