Spurious "Field doesn't have a default value" warning

CREATE TABLE `facebook_friendship` (

 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,

 `facebook_user_id_from` char(36) NOT NULL,

 `user_id_from` char(36) NOT NULL,

 `facebook_user_id_to` char(36) NOT NULL,

 `user_id_to` char(36) NOT NULL,

 `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,

 PRIMARY KEY (`id`),

 UNIQUE KEY `user_id_uq` (`facebook_user_id_from`,`facebook_user_id_to`)

);

MariaDB [test]> insert into `facebook_friendship` (`facebook_user_id_from`,`user_id_from`,`facebook_user_id_to`,`user_id_to`)  values ('walter', 'walter2', 'walter3', 'walte4r');

Query OK, 1 row affected, 1 warning (0.01 sec)

MariaDB [test]> show warnings;

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

| Level   | Code | Message                                         |

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

| Warning | 1364 | Field 'created_at' doesn't have a default value |

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