Wrong result on INSERT..SELECT when converting from SIGNED to UNSIGNED

CREATE OR REPLACE TABLE t1 (a TINYINT UNSIGNED);

CREATE OR REPLACE TABLE t2 (a TINYINT);

INSERT INTO t1 VALUES (255);

INSERT IGNORE INTO t2 SELECT a FROM t1;

SELECT * FROM t2;

+------+

| a    |

+------+

|   -1 |

+------+