mysql> create table a1 (i int, j int);
Query OK, 0 rows affected (0.17 sec)

mysql> create table a2 ( i int , j int, foreign key (i) references a1(i));
ERROR 1005 (HY000): Can't create table '.\test\a2.frm' (errno: 150)
mysql> alter table a1 add index (i);
Query OK, 0 rows affected (0.16 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create table a2 ( i int , j int, foreign key (i) references a1(i));
Query OK, 0 rows affected (0.27 sec)