mysql一个库所有表中没有字段字段A,则给该表添加字段A？

如题：我的存储过程会报一个错误：Procedure execution failed
1265 - Data truncated for column '_tableName' at row 13
下面是存储过程：
BEGIN

DECLARE _tableName VARCHAR(32);
DECLARE _done INT DEFAULT 0;    
DECLARE _cur CURSOR FOR 
        SELECT  table_name FROM information_schema.`COLUMNS`  WHERE TABLE_SCHEMA='test' AND TABLE_NAME NOT LIKE 'ACT%' group by TABLE_NAME ;
DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET _done=1;#错误定义，标记循环结束  
OPEN _cur;
    REPEAT
        FETCH _cur INTO _tableName;
        IF NOT _done THEN
                BEGIN            
                    IF NOT EXISTS (SELECT * FROM information_schema.columns WHERE table_schema = 'test'  AND table_name = _tableName AND column_name = 'PROJECT_ID') THEN
                            SET @STMT := CONCAT("ALTER TABLE ",_tableName," ADD COLUMN PROJECT_ID VARCHAR(32);");
                            PREPARE STMT FROM @STMT;
                            EXECUTE STMT;
                            #ALTER TABLE _tableName ADD COLUMN PROJECT_ID  VARCHAR(32);
                    END IF; 
                END;
        END IF;
    UNTIL _done END REPEAT;
CLOSE _cur;
SET _done = 0;#只有定义为0，新的循环才能继续。

END