UPDATE w/ join against MRG_MyISAM table with read-only sub-table fails

drop table if exists t1, t2, t2_0;

CREATE TABLE `t1` (

  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `a` int(11) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=MyISAM;

CREATE TABLE `t2_0` (

  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `b` int(11) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=MyISAM;

CREATE TABLE `t2` (

  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `b` int(11) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=MRG_MyISAM UNION=(`t2_0`);

FLUSH TABLES;

myisampack -f ./data/test/t2_0

myisamchk -rq ./data/test/t2_0

update t1 join t2 using (id) set t1.a=t2.b;

> update t1 join t2 using (id) set t1.a=t2.b;

ERROR 1036 (HY000): Table 't2_0' is read only