Wrong data type for COALEASCE(?,1) in prepared statements

DROP TABLE IF EXISTS t1;

PREPARE stmt FROM "CREATE TABLE t1 AS SELECT CONCAT(COALESCE(?,1)) AS a, CONCAT(CASE WHEN TRUE THEN ? ELSE 1 END) AS b";

SET @a=1;

EXECUTE stmt USING @a,@a;

SHOW CREATE TABLE t1;

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

| Table | Create Table                                                                                                                                   |

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

| t1    | CREATE TABLE `t1` (

  `a` varbinary(21) DEFAULT NULL,

  `b` varchar(21) CHARACTER SET utf8 DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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