Wrong field type for CAST(temporal_column AS [UN]SIGNED)

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 AS SELECT

  CAST(TIME'10:10:10.123456' AS SIGNED) AS c1,

  CAST(TIME'10:10:10.123456' AS UNSIGNED) AS c2;

DESC t1;

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

| Field | Type                | Null | Key | Default | Extra |

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

| c1    | bigint(17)          | NO   |     | NULL    |       |

| c2    | bigint(20) unsigned | NO   |     | NULL    |       |

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