jQuery Sortable issue in IE9

index.html

<!--
  Error: Object doesn't support property or method 'attr'
-->

<div class='popover' id='playlist_menu'>
  <ul class='toplay' style='overflow:auto'>
    <li class='playlist_item' data-id='27' data-order='1' id='videos_27'></li>
    <li class='playlist_item' data-id='1' data-order='2' id='videos_1'></li>
    <li class='playlist_item' data-id='52' data-order='3' id='videos_52'></li>
  </ul>
</div>

javascript.js

// Error: Object doesn't support property or method 'attr'

var items = $(".popover ul.toplay").sortable("toArray");
var i, n;
for (i = 0, n = items.length; i < n; i++) {
  var item = $(".playlist_item#" + items[i]);
  item.attr("data-order", i + 1); // This is where the error comes from
}