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

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 AS SELECT

  CAST(1.1111111111 AS SIGNED) AS c1,

  CAST(1.1111111111 AS UNSIGNED) AS c2;

DESC t1;

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

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

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

| c1    | bigint(13)          | NO   |     | NULL    |       |

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

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