Item Fulfillment works, but still throws INVALID_TRANS_TYP error.

I have written a restlet which fulfills items of a salesorder. The code is fairly straightforward:

    // Create the item fulfillment record      var child_record = nlapiTransformRecord('salesorder', 11249, 'itemfulfillment')        // Actually fulfill each of the items      var item_count = child_record.getLineItemCount('item');      for (var i = 1; i <= item_count; i++)      {          child_record.setLineItemValue('item', 'itemreceive', i, 'T');          child_record.setLineItemValue('item', 'quantity', i, 1);      }        // Save the fulfillment record      var child_record_id = nlapiSubmitRecord(child_record, true);

This works - the fulfillment item is created, etc etc. But this still throws the following error:

{u'error': {u'code': u'INVALID_TRANS_TYP',              u'message': u'Transaction type specified is incorrect.'}}