Wrong ER_DUP_ENTRY upon ADD UNIQUE KEY on versioned table

CREATE TABLE t1 (i INT) WITH SYSTEM VERSIONING;

INSERT INTO t1 VALUES (1),(1);

DELETE FROM t1;

ALTER TABLE t1 ADD UNIQUE KEY (i);

# Cleanup

DROP TABLE t1;

mysqltest: At line 5: query 'ALTER TABLE t1 ADD UNIQUE KEY (i)' failed: 1062: Duplicate entry '1' for key 'i'