composite type and assignment in plpgsql

what's wrong with this?

create type tSession
    as ( ty_found boolean, ty_Session char(32) );

create or replace function GetSessionID( integer )
    returns tSession as '
declare
    thisSession tSession;
begin
    --HERE!!!
    thisSession := ( ''t'', md5( now( ) || rand( ) ) );
    return thisSession;
end;
' language plpgsql;


thx