Using jQuery TinySort plugin to sort by attribute

Question

I have an unordered list with class name .summary_ul with a number of list items of class .summary_ul_li, each of which has a 'val' attribute whose value is a string of letters. I am calling this function from the javascript file:

function sort_by_val(){
    var $tosort = $('.summary_ul_li');
    $tosort.tsort({attr:"val"});
}

However, $tosort does not sort, and remains in its original order. I am trying to use this plugin:

http://tinysort.sjeiti.com/

Is there something I'm missing here?