[jQuery] sortable serialize doesn't work

[shichuanr]

I have a strange problem with sortable serialize

<ul id="sortable1">
        <li class="ui-state-default">Item 1</li>
        <li class="ui-state-default">Item 2</li>
        <li class="ui-state-default">Item 3</li>
        <li class="ui-state-default">Item 4</li>
        <li class="ui-state-default">Item 5</li>
</ul>
<script type="text/javascript">serial = $('#sortable1').sortable
("serialize"); alert(serial);</script>

To alert sortable serialize doesn't return any value? I received alert
'undefined'. What could be wrong?