1318 - Incorrect number of arguments for PROCEDURE

DROP PROCEDURE `ModificarUsuario`//
CREATE DEFINER=`root`@`localhost` PROCEDURE `ModificarUsuario`(
   IN `Aid` INT,
   IN `Aced` VARCHAR(100),
   IN `Anombre` VARCHAR(100), 
   IN `Acargo` VARCHAR(100), 
   IN `Acedula` VARCHAR(100), 
   IN `Ausuario` VARCHAR(100),
   IN `Apass` VARCHAR(100),
   OUT `res` VARCHAR(10) )
BEGIN
    SELECT COUNT(usuario) INTO res FROM `usuario` WHERE `cedula`=Aced and `id`<>Aid;
    IF  res =0 THEN
       UPDATE `usuario` SET cedula=Aced, nombre=Anombre, cargo=Acargo, usuario=Ausuario, contrasena=Apass WHERE cedula=Acedula;
    END IF;
END

When I use this procedure I get the error "expected 8, got 7." I don't understand this, if we look at the code there are 7 input parameters and one out parameter. It seems that the out parameter needs to be specified as well when calling the procedure, any idea why?