MySQL添加不了外键

用cmd进的MySQL
mysql> create table t_borrow_record1 (
-> borrow_id char(6),
-> reader_id char(6),
-> isbn char(17),
-> borrow_date date,
-> primary key(borrow_id),
-> constraint fk_isbn foreign key(isbn) references t_book(isbn),
-> constraint fk_readerid foreign key(reader_id) references t_reader(reader_id)
-> );
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'fk_readerid' in the referenced table 't_reader'

求大家帮我看一下是啥问题 如何解决
本人刚学这个 实在是解决不了
真心感谢!

大概原因我知道是无法添加外键约束。在引用的表“t_reader”中缺少约束“fk_readerid”的索引

还要主键id不能为空！外键也是