Joined SearchColumn returning error

Hi,

I'm executing a custom search on a custom record. That custom record has a joined record from which I would like to retrieve the fields.
to me, everything looks good, but I get an error when executing this code:

 <span style="color:'#000000'"><span style="font-family:'Times'">error code: UNEXPECTED_ERROR error message:TypeError: Cannot find default value for object. (SYSTEM_LIBS#2117)</span></span> 

The filters select the correct custom record, but when I add the SearchColumns I get the error. Are joins not supported for custom records?

     // SQL Pseudocode      // SELECT customrecord_af_shipment.custrecord_af_shipment_selected_rate.custrecord_af_shipment_carrier_code,      // customrecord_af_shipment.custrecord_af_shipment_selected_rate.custrecord_af_shipment_service_code      // WHERE customrecord_af_shipment.custrecord_af_shipment_transaction = salesOrderId      // AND customrecord_af_shipment.custrecord_af_shipment_source = lineItemLocationId        var salesOrderId = record.getFieldValue('orderid');      var lineItemLocationId = record.getLineItemValue('item', 'location', 1);        filters = [];      filters.push(nlobjSearchFilter('custrecord_af_shipment_transaction', null, 'is', salesOrderId));      filters.push(nlobjSearchFilter('custrecord_af_shipment_source', null, 'is', lineItemLocationId));        columns = [];      columns.push(nlobjSearchColumn('custrecord_af_shipment_carrier_code', 'custrecord_af_shipment_selected_rate', null));      columns.push(nlobjSearchColumn('custrecord_af_shipment_service_code', 'custrecord_af_shipment_selected_rate', null));        searchResults = nlapiSearchRecord('customrecord_af_shipment', null, filters, columns);