Assign a Javascript Value to a label in Gridview....

Microsoft JScript runtime error: Unable to set value of the property 'innerHTML': object is null or undefined

<script language="Javascript" type="text/javascript">
    var grandTotal = 0;
    var percentTotal = 0;
    function multiply2(Rate, number, total) {

            var grid = document.getElementById('GridView5');
            var num = parseFloat(document.getElementById(number).value);
            
            
            var tot = document.getElementById(total);
            var totValue = parseFloat(((Rate * num)));
            var totValueRound = Math.round(totValue);
            var totValueRound = totValue;
            tot.innerHTML = "$" + totValueRound.toFixed(2);
            grandTotal += totValue;
            percentTotal = grandTotal * 10 / 100;
            for (j = 0; j < grid.rows.length - 1; j++) {
            grid.rows[grid.rows.length - 1].cells[2].children[0].innerHTML = "$" + grandTotal.toFixed(2);
        }
        document.getElementById('lblTotal').innerHTML = percentTotal.toFixed(2);
       
    } 
</script>