Auto-increment does not work with DESC on MERGE table

create table t (a int not null, primary key(a desc)) engine=MyISAM;

create table tm (a int not null auto_increment, primary key(a desc)) engine=MERGE union=(t) insert_method=FIRST;

insert into tm () values ();

insert into tm () values ();

insert into tm () values ();

drop table tm, t;

At line 6: query 'insert into tm () values ()' failed: ER_DUP_ENTRY (1062): Duplicate entry '2' for key 'PRIMARY'