Inconsistent Alter column / modify column behavior

CURRENT_TIMESTAMP can be set as DEFAULT with MODIFY but not with ALTER:

> ALTER TABLE attachment_version ALTER COLUMN created_at SET DEFAULT CURRENT_TIMESTAMP;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'CURRENT_TIMESTAMP' at line 1

> ALTER TABLE attachment_version MODIFY COLUMN created_at timestamp DEFAULT CURRENT_TIMESTAMP;

Query OK, 0 rows affected (0.02 sec)

The table in example is InnoDB.