How to make a projection function parameter optional?

I have a projection function which looks like below.

function GetInvoiceList List<Structure(ListInvoiceResponse)> {
   parameter CompanyId Text;
   parameter ContactPersonId Text;
   parameter CustId Text;
   parameter InvoiceId Text {
      required = [false];
   }
   parameter FromDate Date;
   parameter ToDate Date;
   parameter PageSize Integer;
   parameter PageNumber Integer;
}

As you can see InvoiceId has required marked as false. However, when I deploy this function, API explorer documentation still shows InvoiceId as required parameter.

How do I make a paremeter optional?