jQuery UI: sortable('toArray') returns an empty array

This has me stumped. The follow code returns ",,,,,,":

<script type="text/javascript">
$(function() {
    $('#listB').sortable({
        connectWith: '#listA',
        update: function(event, ui) {
            var result = $(this).sortable('toArray');
            alert(result);
            }
    });
    $('#listA').sortable({
        connectWith: '#listB'
    });
});
</script>

<div id="boxA">
    <ul id="listA" class="myList">
        <li value="1">Item A</li>
        <li value="2">Item B</li>
        <li value="3">Item C</li>
        <li value="4">Item D</li>
        <li value="5">Item E</li>
        <li value="6">Item F</li>
        <li value="7">Item G</li>
    </ul>
</div>

<div id="boxB">
    <ul id="listB" class="myList">
        <li value="1">Item A</li>
        <li value="2">Item B</li>
        <li value="3">Item C</li>
        <li value="4">Item D</li>
        <li value="5">Item E</li>
        <li value="6">Item F</li>
        <li value="7">Item G</li>
    </ul>
</div>

Why?! It's driving me insane! Any suggestions?