通过PDO调用存储过程时，不保存MySQL ENUM值

Question

I have created this stored procedure:

CREATE PROCEDURE LogAdd(
    IN log_title VARCHAR(128),
    IN log_type ENUM('message','warning','error'),
    IN log_info TEXT
) 
DETERMINISTIC 
MODIFIES SQL DATA 
COMMENT 'Insert a new log entry' 
BEGIN
    INSERT INTO `tbl_logs` (
        `title`,
        `type`,
        `info`
    )
    VALUES (
        log_title,
        log_type,
        log_info
    );
END

which works when executed with data directly added in the query. But when I bind parameters, the type column, which is of ENUM type, is not saved (is empty). I can see that the value is sent as a string:

$stmt = $pdo -> prepare("CALL LogAdd(?, ?, ?)");
$stmt -> bindParam(1, 'Message title');
$stmt -> bindParam(2, 'message');
$stmt -> bindParam(3, 'Message information');

$stmt -> execute();

Why isn't the ENUM value saved?