Bogus ER_TABLE_EXISTS_ERROR upon an attempt to add a foreign key

--source include/have_innodb.inc

CREATE TABLE t1 (f1 INT, f2 INT, KEY(f1), KEY(f2)) ENGINE=InnoDB;

CREATE TABLE t2 (a INT, b INT, KEY(b)) ENGINE = InnoDB;

ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY (a) REFERENCES t1(f1) ON DELETE SET NULL;

SET foreign_key_checks= OFF;

ALTER TABLE t2 MODIFY a CHAR(1) NOT NULL;

SET foreign_key_checks= ON;

ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY (b) REFERENCES t1 (f2);

# Cleanup

DROP TABLE t2, t1;

mysqltest: At line 12: query 'ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY (b) REFERENCES t1 (f2)' failed: 1050: Table './test/t2' already exists