Price Level always returns 'null'

I am developing a Script to populate a specific price level for all items on sales order.

As a first step, I tried to read the current price level for the first item on the sales order. I created a custom form and triggered the following script using a button on the form:

 function setLineField(type)   {   nlapiSelectLineItem('item', 1);  firstPriceLevel = nlapiGetCurrentLineItemValue('item', 'pricelevel');  alert (firstPriceLevel);  }

This always returns 'null'. I tried the same code on other fields, and it returns the correct value. I tried it on multiple sales orders, with price level set to the defaults and otherwise, still returns null.