Disable gross amt field with script

Hi Guys

I am trying to disable salesmen from editing the "gross amt" field in a script but I cant seem to get it right.

I have a script to disable the "amount" field which looks like this:

                 var itemlist = form.getSubList('item');
  var salesAmount = itemlist.getField('amount');
  salesAmount.setDisplayType('disabled');

I have tried to take the same approach for the gross amount field like this:

                var itemlist = form.getSubList('item');
  var grossAmount = itemlist.getField('grossAmt');
  grossAmount.setDisplayType('disabled');

but it doesnt block the field from being edited. Is there something im missing, does the gross amount field sit in the sublist or is the reference wrong?