alter table add foreign key fails

I have 3 tables and they all have innodb engine:

video(url, title, desc, country,...) url -> primary key
videoCat(_url, category) {_url,category} -> primary key
favorite(fav_url, thumb_path) fav_url -> primary key

then I do:

alter table favorite
add foreign key(fav_url) references video(url)
on delete cascade

and everything goes smooth, but when I try:

alter table videoCat
add foreign key(_url) references video(url)
on delete cascade

I get:

1452 - Cannot add or update a child row: a foreign key constraint fails (bascelik_lookaroundyou., CONSTRAINT #sql-efa_1a6e91a_ibfk_1 FOREIGN KEY (_url) REFERENCES video (url) ON DELETE CASCADE)

why???

p.s. I am using phpmyadmin ver. 3.3.9.2