Wrong result while using index for group-by

--source include/have_innodb.inc

create table t (pk int primary key, a int, key(a)) engine=InnoDB;

insert into t values (1,NULL),(2,8),(3,5),(4,NULL),(5,10);

SELECT MIN(pk), a FROM t WHERE pk <> 1 GROUP BY a;

# Cleanup

drop table t;

--source include/have_innodb.inc

create table t (pk int primary key, a int, key(a)) engine=InnoDB;

insert into t values (1,NULL),(2,8),(3,5),(4,NULL),(5,10);

SELECT MIN(pk), a FROM t WHERE pk <> 1 GROUP BY a;

# Cleanup

drop table t;

SELECT MIN(pk), a FROM t WHERE pk <> 1 GROUP BY a;

MIN(pk)	a

3	5

2	8

5	10

EXPLAIN EXTENDED SELECT MIN(pk), a FROM t WHERE pk <> 1 GROUP BY a;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra

1	SIMPLE	t	range	PRIMARY	a	5	NULL	6	100.00	Using where; Using index for group-by

Warnings:

Note	1003	select min(`test`.`t`.`pk`) AS `MIN(pk)`,`test`.`t`.`a` AS `a` from `test`.`t` where `test`.`t`.`pk` <> 1 group by `test`.`t`.`a`