Problems with Custom Columns

I am using columns to return specfic results, and it works fine until I add in custom columns, then it throws an error. Has anyone else tried this and had the same problem?

the code snipets below

Thanks
RIchard

var columns = new Array();
columns[0] = new nlobjSearchColumn('internalid');
columns[1] = new nlobjSearchColumn('cost');
columns[3] = new nlobjSearchColumn('custitemshipper');
columns[4] = new nlobjSearchColumn('custitemexrate');
columns[5] = new nlobjSearchColumn('custitemmarginrrp');
columns[6] = new nlobjSearchColumn('custitemusdpurchase');


var searchresults = nlapiSearchRecord('inventoryitem',null,filters,columns);