serialize() Sortable Accordion

serialize() Sortable Accordion

I've created a sortable accordion but I can't seem to figure out how to serialize the indexes like you can on a non-accordion sortable. 

on my H3 elements I have the id="agendas_1234" and then the following code.  But the line where I define the params variable doesn't work.  I don't get the same [1234,3456,123] that I would get with a regular sortable (no accordion)
  1. $(tab).find('#accordion').accordion({
            header: "> div > h3",
            collapsible: true
        }).sortable({
            axis: "y",
            handle: "h3",
            stop: function(event, ui) {
                stop = true;
            },
            update: function(event, ui) {
                var params = $(this).sortable('serialize');
            }
        });