My search is not working

var shipad = "FedEx";
var search =  nlapiSearchRecord('shipitem', null , new nlobjSearchFilter('itemid', null , 'is', shipad));
	if(search != null){
	   var shipID = search[0].getId();
	} 
	nlapiLogExecution('DEBUG', 'amount:',shipID);

I keep getting this error "An nlobjSearchFilter contains an invalid operator, or is not in proper syntax: itemid."