Table disappear after two alter table command

CREATE TABLE `parent1` (

  `id1` smallint(5) unsigned NOT NULL AUTO_INCREMENT,

  PRIMARY KEY (`id1`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `parent2` (

  `id2` smallint(5) unsigned NOT NULL AUTO_INCREMENT,

  PRIMARY KEY (`id2`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `child` (

  `id1` smallint(5) unsigned NOT NULL,

  `id2` smallint(5) unsigned NOT NULL,

  PRIMARY KEY (`id1`,`id2`),

  UNIQUE KEY `id2` (`id2`) USING BTREE,

  CONSTRAINT `child_ibfk_1` FOREIGN KEY (`id2`) REFERENCES `parent2` (`id2`) ON DELETE CASCADE,

  CONSTRAINT `child_ibfk_2` FOREIGN KEY (`id1`) REFERENCES `parent1` (`id1`) ON DELETE CASCADE

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

MariaDB [LUFI]> SET FOREIGN_KEY_CHECKS=0;

Query OK, 0 rows affected (0.001 sec)

MariaDB [LUFI]> ALTER TABLE `child` DROP PRIMARY KEY;

Query OK, 0 rows affected (0.276 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [LUFI]> ALTER TABLE `child` DROP INDEX `id2`;

ERROR 1025 (HY000): Error on rename of './LUFI/#sql-511_321' to './LUFI/child' (errno: 150 "Foreign key constraint is incorrectly formed")

MariaDB [LUFI]> SHOW TABLES; 

+----------------+

| Tables_in_LUFI |

+----------------+

| parent1        |

| parent2        |

+----------------+

2 rows in set (0.002 sec)

MariaDB [LUFI]> CREATE TABLE `child` (

    ->   `id` smallint(5) unsigned NOT NULL

    -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

ERROR 1005 (HY000): Can't create table `LUFI`.`child` (errno: 150 "Foreign key constraint is incorrectly formed")

MariaDB [LUFI]>