innodb_force_primary_key option does not force PK or unique key

MariaDB [test]> set global innodb_force_primary_key = 1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> create table t1 (i int not null);

ERROR 1173 (42000): This table type requires a primary key

MariaDB [test]> # works

MariaDB [test]> create table t1 (i int not null, key(i));

Query OK, 0 rows affected (0.61 sec)

MariaDB [test]> # but it's not a primary key or unique key

MariaDB [test]> insert into t1 values (1),(1);

Query OK, 2 rows affected (0.08 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> show create table t1 \G

*************************** 1. row ***************************

       Table: t1

Create Table: CREATE TABLE `t1` (

  `i` int(11) NOT NULL,

  KEY `i` (`i`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)