sortable("serialize", {key: "foo[]"}) returns an empty string for an empty list

The serialize method returns an empty String for empty lists when a custome key is defined. A form/ajax submittable string should rather be 'foo[]=' in the following example, if option_1 is moved to the connected list #bar:

<ul id="foo" class="sortable">
  <li id="option_1">1</li>
</ul>
<ul id="bar" class="sortable">
  <li id="option_2">2</li>
</ul>

<script type="text/javascript">
  $(function(){
    $('.sortable').sortable({
      connectWith: '.sortable',
      update: function (event, ui) {
        console.log('Update: ' + 
          $(event.target).sortable('serialize', {
            key: event.target.id + '[]'
          })
        );
      }
    });
  });
</script>