Error Function doen't exist in trigger

SET NAMES utf8;

SET time_zone = '+00:00';

SET foreign_key_checks = 0;

SET sql_mode = 'NO_AUTO_VALUE_ON_ZERO';

DROP TABLE IF EXISTS `t1`;

CREATE TABLE `t1` (

  `col1` int(11) NOT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `t1` (`col1`) VALUES (3);

DELIMITER ;;

CREATE TRIGGER `t1_bu` BEFORE UPDATE ON `t1` FOR EACH ROW

BEGIN 

    DECLARE TEST INT(11) DEFAULT 0;

SET TEST=yopyop();

END;;

DELIMITER ;

DROP TABLE IF EXISTS `t2`;

CREATE TABLE `t2` (

  `col1` int(11) NOT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `t2` (`col1`) VALUES

(3);

UPDATE t1,t2 SET t1.col1=4 WHERE t1.col1=3 AND t1.col1=t2.col1; 

Error (1305): FUNCTION yop.yopyop does not exist