Unexpected ER_INVALID_USE_OF_NULL upon concurrent UPDATE and ALTER

--source include/have_innodb.inc

--connect (con1,localhost,root,,test)

--connection default

CREATE  TABLE t1 (a INT, b INT) ENGINE=InnoDB;

ALTER TABLE t1 DROP COLUMN a;

INSERT INTO t1 (b) VALUES (NULL);

SET SQL_MODE= '';

--send

ALTER TABLE t1 ADD PRIMARY KEY (b) ;

--connection con1

PREPARE stmt FROM "UPDATE t1 SET b = 1";

EXECUTE stmt;

# Cleanup

--disconnect con1

--connection default

--reap

DROP TABLE t1;

mysqltest: At line 20: query 'reap' failed: 1138: Invalid use of NULL value