mysql 全文索引查找不出来

ngram_token_size 设置为 1

CREATE TABLE articles (
      id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
      title VARCHAR(200),
      FULLTEXT (title) WITH PARSER ngram
    ) ENGINE=InnoDB CHARACTER SET utf8mb4;

INSERT INTO articles (title) VALUES("a");
INSERT INTO articles (title) VALUES("b");
INSERT INTO articles (title) VALUES("f");

搜索 a，返回空。为什么？

select * FROM  articles  WHERE   MATCH (title) AGAINST ('+a' IN BOOLEAN MODE) ;

ps， 把 a 改成 b 或者 f 则能正常返回。