CAST(COALESCE(year_field)) returns wrong value

CREATE OR REPLACE TABLE t1 (a YEAR);

INSERT INTO t1 VALUES (2000);

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

SHOW WARNINGS;

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

| CAST(a AS DATE) | CAST(COALESCE(a) AS DATE) |

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

| 2000-00-00      | NULL                      |

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

1 row in set, 1 warning (0.00 sec)

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

| Level   | Code | Message                          |

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

| Warning | 1292 | Incorrect datetime value: '2000' |

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

bool Type_handler_year::Item_get_date(Item *item, MYSQL_TIME *ltime,

                                             ulonglong fuzzydate) const

{

  return item->get_date_from_year(ltime, fuzzydate);

}