Cannot access empty property in

Får den her fejl:
Fatal error: Cannot access empty property in /home/docs/class.calendar.inc.php on line 96


//Initialize the constructor of the class with calendar date , tempalte html with smart tags, stylesheet and prefix to add for stylesheet.
    function calendar( $date , $tempalate , $stylesheet , $stylesheet_prefix ){
        $this-> $cal_s_date = $this-> set_calendar_show_date ($date);
        $this-> $calendar_calendar_stylesheet = $this-> set_calendar_stylesheet ( $stylesheet  );
        $this-> $cal_ssht_pfx = $this-> set_calendar_stylesheet_prefix ( $stylesheet_prefix );
        $this-> $calendar_calendar_tempalate = $this-> set_calendar_tempalate ( $tempalate );
    }

linie 96 er:
$this-> $cal_s_date = $this-> set_calendar_show_date ($date);