Convert sales order to item fulfillment

I am trying to convert sales orders to item fulfillments. Included is the code I am trying to run and the errors thrown. If anyone has any idea on how to fix, I would very much appreciate it!

var if_rec = nlapiTransformRecord('salesorder', so_id, 'itemfulfillment',true); //using dynamic or standard results in same error

if_rec.setFieldValue('status', 'B');

var if_id = nlapiSubmitRecord(if_rec);

Results in the following error: Code: USER_ERROR Details: You must enter at least one line item for this transaction.

However, if I try:

var if_rec = nlapiTransformRecord('salesorder', so_id, 'itemfulfillment');

if_rec.setFieldValue('status', 'B'); //SET TO PACKED