Alert on Item selection

This should be fairly straightforward, but I can't seem to figure it out.

I'm trying to display an alert when a user selects particular items on a quote.

In the code below, I'm just trying to display the value of the item.  The alert only comes up when I change the custcol_item_number field, but it returns NULL.

function myValidateField(type, name, linenum) {   if (type == 'item' && name == 'custcol_item_number'){    alert(nlapiGetLineItemValue('item','custcol_item_number', linenum));   }   return true;  }