ALTER TABLE DROP CONSTRAINT does not work for foreign keys

MariaDB [test]> create table tpk(id int primary key, c1 int);

Query OK, 0 rows affected (0.646 sec)

MariaDB [test]> create table tfk(id int primary key, c1 int, constraint `_________fk_transaction_111_11111_1111_111_id` FOREIGN KEY (`c1`) REFERENCES tpk (`id`)

 ON DELETE NO ACTION ON UPDATE NO ACTION);

Query OK, 0 rows affected (0.416 sec)

MariaDB [test]> show create table tfk\G

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

       Table: tfk

Create Table: CREATE TABLE `tfk` (

  `id` int(11) NOT NULL,

  `c1` int(11) DEFAULT NULL,

  PRIMARY KEY (`id`),

  KEY `_________fk_transaction_111_11111_1111_111_id` (`c1`),

  CONSTRAINT `_________fk_transaction_111_11111_1111_111_id` FOREIGN KEY (`c1`)

REFERENCES `tpk` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.007 sec)

MariaDB [test]> alter table tfk drop constraint `_________fk_transaction_111_111

11_1111_111_id`;

ERROR 1091 (42000): Can't DROP CONSTRAINT `_________fk_transaction_111_11111_111

1_111_id`; check that it exists

MariaDB [test]> alter table tfk drop foreign key `_________fk_transaction_111_11

111_1111_111_id`;

Query OK, 0 rows affected (0.129 sec)

Records: 0  Duplicates: 0  Warnings: 0