Can not insert null by fixtures

Rails
1

I have the following a table and a fixture(csv file).

create table aaaa (
id mediumint unsigned not null auto_increment,
name varchar(192) not null,
parent_id mediumint unsigned null,
note varchar(192) not null,
primary key(id)
)
TYPE=InnoDB;

id,name,parent_id,note
1,Admin Group,""
2,Guest Group,""

I expected that null was inserted to parent_id,
but 0 was inserted.

Please help.

Junichi.