Cannot write to PO Rate field

Hi,

Seems to be an issue writing to this field on an invoice form.
This is a simple client side script which is executed upon save record.

Here is my code:


function saverecord()
{

var comm = nlapiGetFieldText('custbodyagentcommtrans');
comm = parseInt(comm)/100;


for(i=1; i<= nlapiGetLineItemCount('item'); i++)
{



//nlapiSelectLineItem('item', i);

var therate = parseFloat(nlapiGetLineItemValue('item', 'rate', i));
var theqty = nlapiGetLineItemValue('item', 'quantity', i);
var itempercent = parseFloat(nlapiGetLineItemValue('item', 'custcolitemcostpercent', i));


var calcamt = parseFloat(((therate) - (therate*comm)) * (itempercent/100));

//alert(calcamt);
//alert(nlapiGetLineItemValue('item', 'porate', i));
nlapiSetLineItemValue('item', 'porate', i,  calcamt );

}

return true;
}

I am able to read the field value of PO Rate but I can’t to write to it.