如何在 MariaDB 或 MySQL 中创建文本类型的外键?

我有两张桌子:

CREATE TABLE first_table(
my_id TEXT(6) NOT NULL,
content VARCHAR(30) NOT NULL,
PRIMARY KEY(my_id(6))
) Engine=InnoDB charset utf8mb4 collate utf8mb4_general_ci;

CREATE TABLE second_table(
another_id TEXT(6) NOT NULL,
my_id TEXT(6) NOT NULL,
another_content VARCHAR(30) NOT NULL,
PRIMARY KEY(another_id(6))
) Engine=InnoDB charset utf8mb4 collate utf8mb4_general_ci;

但在第二个表中我无法创建引用第一个表的外键，首先我尝试过:

ALTER TABLE second_table
ADD FOREIGN KEY (my_id)
REFERENCES first_table(my_id)
ON DELETE CASCADE ON UPDATE CASCADE;

ERROR 1170 (42000): BLOB/TEXT column 'my_id' used in key specification without a key length
MariaDB [base_ventas]>

ALTER TABLE second_table
ADD FOREIGN KEY (my_id(6))
REFERENCES first_table(my_id)
ON DELETE CASCADE ON UPDATE CASCADE;

ERROR 1005 (HY000): Can't create table `base_ventas`.`#sql-1a08_23c` 
(errno: 150 "Foreign key constraint is incorrectly formed")