Script to count line items on a Purchase order

I am a novice when it comes to writing code. I have a script that I use to (1) count the line item QTY on a purchase order and (2) count the QTY billed on a bill.

The code works fine until I add a "subtotal" line or a "discount" line to the purchase order. Then I receive the following result "NaN"

How do I exclude non-inventory line from the count?

function calculatePOqty (type, name)
{
var c = nlapiGetLineItemCount('item');
    var itemCount = 0;
   
    for (var i = 1; i <= c; i++) {
        itemCount += parseInt(nlapiGetLineItemValue('item', 'quantity', i), 10);   
    }

    nlapiSetFieldValue('custbody_total_qty', itemCount);
}

function calculateBillqty (type, name)