null returned from nlapiSearchRecord

I've been off in C# land for too long.  Can someone please tell me why nlapiSearchRecord is returning null?

function run(portlet, column)
{
var filters = new Array();
filters[0] = new nlobjSearchFilter('type', null, 'is', ['_salesorder']);

var columns = new Array();
columns[0] = new nlobjSearchColumn('internalid');

var results = nlapiSearchRecord('transaction', null, filters, columns);
nlapiLogExecution('DEBUG','Trace','Number of records = ' + results.length);

for (var i = 0; i < results.length; i++)
{
  var row = results[i];
  var cols = row.getAllColumns();
 
  nlapiLogExecution('DEBUG','Trace','Internal ID = ' + row.getValue(cols[0]));
}
}

Thanks.