DATE(zerofill_column) and DATE(COALESCE(zerofill_column)) return different results

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a INT(6) ZEROFILL);

INSERT INTO t1 VALUES (1);

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

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

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

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

| 2000-00-01 | NULL              |

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

1 row in set, 1 warning (0.00 sec)