DATE(COALESCE(year_column)) returns a wrong result

CREATE OR REPLACE TABLE t1 (a YEAR);

INSERT INTO t1 VALUES (NULL);

SELECT COALESCE(a), DATE(COALESCE(a)) FROM t1;

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

| COALESCE(a) | DATE(COALESCE(a)) |

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

|        NULL | 0000-00-00        |

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