index not defined as NOT NULL

[off mct]

I created a table in host apple:
CREATE TABLE `NullIndex` (
`id` int(10) unsigned NOT NULL auto_increment,
`a` int(11) default NULL,
`b` varchar(100) NOT NULL default '',
PRIMARY KEY (`id`),
KEY `a` (`a`)
) ENGINE=MyISAM;

Then, I created a federated table in another machine:
CREATE TABLE `NullIndex` (
`id` int(10) unsigned NOT NULL auto_increment,
`a` int(11) default NULL,
`b` varchar(100) NOT NULL default '',
PRIMARY KEY (`id`),
KEY `a` (`a`)
) ENGINE=FEDERATED
COMMENT='mysql://devel:devel@apple/test/NullIndex';

This produced an error:
ERROR 1121: Column 'a' is used with UNIQUE or INDEX but is not defined as NOT NULL

I'm not sure why this happened but I was able to solve the problem by removing "KEY `a` (`a`)" in the federated table DDL such that:
CREATE TABLE `NullIndex` (
`id` int(10) unsigned NOT NULL auto_increment,
`a` int(11) default NULL,
`b` varchar(100) NOT NULL default '',
PRIMARY KEY (`id`)
) ENGINE=FEDERATED
COMMENT='mysql://devel:devel@apple/test/NullIndex';

I don't exactly know what are the drawbacks of this workaround but maybe somebody has the proper solution for this.

Thanks a lot.