Multiple parameters passed from Reporting Services to an Oracle database

I have an oracle procedure that works fine with reporting services if I want either 'All' of the items in the list
or if I want just one, but not multiples.  I have tried creating a function from code I found but I get
an error "Error(4,23): PLS-00201: identifier '','' must be declared".  I have done very little work with either
Oracle or reporting services.  Can someone please give me very clear detailed instructions on
what I'm doing wrong.  Thank you.  My deadline is tomorrow.

create or replace function MultiParam
(
    p_cursor sys_refcursor,
    p_del varchar2 := "','"
) return varchar2
is
    l_value   varchar2(32767);
    l_result  varchar2(32767);
begin
    loop
        fetch p_cursor into l_value;
        exit when p_cursor%notfound;
        if l_result is not null then
            l_result := l_result || p_del;
        end if;
        l_result := l_result || l_value;
    end loop;
    return l_result;
end MultiParam;