Error while removing the constraint

CREATE TABLE A (

  id bigint NOT NULL AUTO_INCREMENT PRIMARY KEY

);

CREATE TABLE B (

  what tinyint NOT NULL,

  a bigint NOT NULL,

  CONSTRAINT x FOREIGN KEY (a) REFERENCES A(id),

  CONSTRAINT y CHECK (what > 0)

);

-- Uncommenting the next line "fixes" an error

-- INSERT INTO И VALUES (1,1);

SELECT b.a FROM B b;

-- the next statement produces an error

-- Unknown column '`db_name`.`b`.`what`' in 'CHECK'

ALTER TABLE B DROP CONSTRAINT x;