Name not ID returned using group in search

I am trying to accumulate some totals from Invoice transaction and then update a custom record with these totals. The problem that I am having is that when I group by postingperiod, it returns the description for the posting period and not the internal id. I need the internal id to search for my custom record. When I don't group by posting period it returns the internal id.

This is my search without group by:

 var columns = new Array();   columns[0] = new nlobjSearchColumn('item');   columns[1] = new nlobjSearchColumn('salesteammember');   columns[2] = new nlobjSearchColumn('postingperiod');   var filters = new Array();   filters[0] = new nlobjSearchFilter( 'mainline', null, 'is', 'F', null);   var searchresults = nlapiSearchRecord( 'invoice', null, filters, columns);   for ( var i = 0; searchresults != null && i < searchresults.length ; i++ )   {    var item = searchresults[i].getValue('item');    var salesrep = searchresults[i].getValue('salesteammember');    var period = searchresults[i].getValue('postingperiod');    nlapiLogExecution ('DEBUG', 'Invoice: ', 'item: ' + item + ' salesrep: ' + salesrep + ' period: ' + period);   }