problems with changing row with draw ()

I have a problem on changing row from data that come back from MySQL update I tried with the draw () function but I get this error:

datatables.min.js:129 Uncaught TypeError: Cannot set property '_aData' of undefined
at s.<anonymous> (datatables.min.js:129)
at s.data (datatables.min.js:115)
at Object.<anonymous> (functions.js:76)
at Function.each (jquery.min.js:2)
at Object.success (functions.js:58)
at i (jquery.min.js:2)
at Object.fireWith [as resolveWith] (jquery.min.js:2)
at y (jquery.min.js:4)
at XMLHttpRequest.c (jquery.min.js:4)

my script and this:

$.ajax({
type: "POST",
url: url,
data: form.serialize(),
success: function(data)
{
var json_txt = $.parseJSON(data);
$.each(json_txt, function(i, item) {

          id_day = item.id_day;
          portata = item.nome_port;
          piatto = item.nome_pia;
          descrizione = item.dsc_piatti;
          ingredienti = item.ingred;
          eng_piatti = item.ing_piatti;                

            var table = $('#example').DataTable();
                var tableRow = table.row( $('#update_table_' + id_day ) );
                var rData = [
                    portata,
                    piatto,
                    descrizione,
                    ingredienti,
                    eng_piatti,
                ]; 
                table.row( tableRow ).data( rData ).draw();

         });
        }
     });    

I do not understand the mistake you can help me?

P.S. sorry for my english