Query empty when in subquery field has null

SET NAMES utf8;

SET time_zone = '+00:00';

SET foreign_key_checks = 0;

SET sql_mode = 'NO_AUTO_VALUE_ON_ZERO';

DROP TABLE IF EXISTS `a1`;

CREATE TABLE `a1` (

  `year` smallint(5) unsigned DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `a1` (`year`) VALUES

(2014), (2015), (2016), (2017), (NULL);

DROP TABLE IF EXISTS `a2`;

CREATE TABLE `a2` (

  `year` smallint(5) unsigned NOT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `a2` (`year`) VALUES

(2014), (2015), (2016), (2017), (2018);

SELECT * FROM `a2`

WHERE year NOT IN (SELECT year FROM a1);