GROUP BY not using index

select max(dt1) from gb1 group by nr;

CREATE TABLE `gb1` (

    `dt1` datetime DEFAULT NULL,

    `nr` int(11) DEFAULT NULL,

    PRIMARY KEY (`nr` ASC , `dt1` ASC)

)  ENGINE=InnoDB DEFAULT CHARSET=latin1;

insert ignore into gb1(dt1,nr) values(now() - interval 1 second,rand()*1000);

/*20 x*/ insert ignore into gb1(dt1,nr) select  dt1 - interval rand() * 10000 second, rand()*1000 from gb1;

explain select max(dt1) from gb1 group by nr;

explain select max(dt1) from gb1 use key for group by(PRIMARY) group by nr;