Client SuiteScript on line item throws NULL

Hi,

I have a client suite script that collects informations of three different item fields and should place them together in one column field on a transaction after entering an the item. I chose validateLine to place the action and the script looks like this:
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function validateLine(type)
{
// Only validate the 'item' sublist
if (type == 'item')
{
  var valueA = nlapiGetCurrentLineItemValue('item', 'custitemharmonized');
  var valueB = nlapiGetCurrentLineItemValue('item', 'custitem29');
  var result = valueA + '\n' + valueB;
  nlapiSetCurrentLineItemValue('item', 'custcol34', result);
  }

return true;
}
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After entering an item and adding the line I get just this on my column field:

NULL
NULL

Any Idea what I'm doing wrong?