erro ao executar sp

CREATE PROCEDURE HORASTRAB (@HRSTRAB CHAR(7) OUTPUT)
AS
BEGIN
DECLARE @HRS DATETIME
DECLARE @MINUTOS DATETIME

  SET @HRS     = (SELECT DATEDIFF(HOUR,rd_hrent,rd_hrsai)from registro_diario
                  group by rd_hrent,rd_hrsai) 
  SET @MINUTOS = (SELECT DATEDIFF(MINUTE,rd_hrent,rd_hrsai)from registro_diario
                  group by rd_hrent,rd_hrsai) 
  SET @HRSTRAB = (CONVERT(CHAR,@HRS)+´:´+CONVERT(CHAR,@MINUTOS))

END
 RETURN @HRSTRAB

Server: Msg 201, Level 16, State 2, Procedure HORASTRAB, Line 0
Procedure ´HORASTRAB´ expects parameter ´@HRSTRAB´, which was not supplied.