Draggable error in function revert with helper defined as function

If I define helper option like this:

$('.items').draggable({
  appendTo: $(document.body),
  helper: function() { 
    $('#proxy').remove();  
    return $('<div id="proxy">proxy...</div>');
  }
});

I get error (when dragging is finished and helper is to be reverted) in file ui.draggable.ext.js on line 85 (o.po is not defined), the quick fix is to modify lines 85, 86 like this:

var nl = rpos.left-(o.po ? o.po.left : 0)-o.margins.left;
var nt = rpos.top-(o.po ? o.po.left : 0)-o.margins.top;

but I dont have necessary insight...