Stored procedure to check if username exists

I'm trying to write a stored procedure

So far I have this, but it always returns 1

CREATE DEFINER=`root`@`localhost` PROCEDURE `USER_EXISTS`(IN `USERNAME` VARCHAR(64) CHARSET utf8mb4)
SELECT CASE WHEN MAX(user_id) IS NULL THEN '0' ELSE '1' END User_exists
  FROM `dbname`.`tablename`
 WHERE `username` = USERNAME

The goal is to perform this via AJAX so the user can see whether registration is likely to succeed before submitting their registration form