Unexpected ER_WRONG_NAME_FOR_INDEX upon adding primary key on top of unique

--source include/have_innodb.inc

CREATE TABLE t1 (pk INT NOT NULL, UNIQUE (pk)) ENGINE=InnoDB;

ALTER TABLE t1 ADD PRIMARY KEY (pk);

# Cleanup

DROP TABLE t1;

mysqltest: At line 4: query 'ALTER TABLE t1 ADD PRIMARY KEY (pk)' failed: 1280: Incorrect index name 'pk'