footerCallback

I have some Problem to add a summary to the footer at the end of the Table. I use your example. The sum of the columns work. I see the right result in the console.log(). Please help me to find the Problem why the result are not shown at the end of the Table.
footerCallback:

            footerCallback: function(tfoot, data, start, end, display){
                var api = this.api();

                $(api.column(5).footer()).html(
                    api.column(5).data().reduce(function(a,b){
                        return a + b;
                    }, 0)
                );
            }

The debugger link is debug.datatables.net/atefic

Anyone a idea what is wrong in my code?

Andreas