Type NSJSONWritingOptions cannot conform to protocol NilLiteralConvertible

at var options = prettyPrinted …：

func JSONStringify(value: AnyObject,prettyPrinted:Bool = false) -> String {

    var options = prettyPrinted ? NSJSONWritingOptions.PrettyPrinted : nil

    if NSJSONSerialization.isValidJSONObject(value) {
        do{
            let data = try NSJSONSerialization.dataWithJSONObject(value,options: options)
            if let string = NSString(data: data,encoding: NSUTF8StringEncoding) {
                return string as String
            }
        } catch {

        }
    }
    return ""
}