command line - mysql errno 150 when creating foreign key -

i'm getting errno 150 , can't figure out why. i've read million other answers must have index set on parent column , i'm not sure why i'm still getting error.

parent table indexes:

mysql> show indexes plans; +-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | table | non_unique | key_name       | seq_in_index | column_name | collation | cardinality | sub_part | packed | null | index_type | comment | index_comment | +-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | plans |          0 | primary        |            1 | plan_id     |         |          86 |     null | null   |      | btree      |         |               | | plans |          0 | plan_id_unique |            1 | plan_id     |         |          86 |     null | null   |      | btree      |         |               | | plans |          1 | plan_id        |            1 | plan_id     |         |          86 |     null | null   |      | btree      |         |               | +-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 3 rows in set (0.08 sec) 

child table indexes:

mysql> show indexes promos; +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | table  | non_unique | key_name | seq_in_index | column_name | collation | cardinality | sub_part | packed | null | index_type | comment | index_comment | +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | promos |          0 | primary  |            1 | promo_id    |         |           0 |     null | null   |      | btree      |         |               | | promos |          1 | plan_id  |            1 | plan_id     |         |           0 |     null | null   |      | btree      |         |               | +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 2 rows in set (0.06 sec) 

error:

mysql> alter table promos add foreign key (plan_id) references plans (plan_id) on delete cascade on update cascade; error 1005 (hy000): can't create table 'dev.#sql-2ce_599a' (errno: 150) 

you have have same collation , character set both columns.

this works: http://sqlfiddle.com/#!2/0f917