manually add foreign key

[Manah Manah]

HI. i'm trying to add a foreign key to existing tables. i get error message. below is my tables and the error message.

how to fix this please?

mysql> show tables;
+--------------------+
| Tables_in_customer |
+--------------------+
| cust_table |
| prod_table |
| sales_table |
+--------------------+

mysql> desc cust_table;
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| cust_id | int | NO | PRI | NULL | auto_increment |
| last_name | varchar(45) | YES | | NULL | |
| first_name | varchar(45) | YES | | NULL | |
| address | varchar(45) | YES | | NULL | |
| email | varchar(45) | YES | | NULL | |
+------------+-------------+------+-----+---------+----------------+

mysql> desc prod_table;
+--------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+----------------+
| prod_id | int | NO | PRI | NULL | auto_increment |
| prod_name | varchar(45) | YES | | NULL | |
| prod_details | varchar(45) | YES | | NULL | |
+--------------+-------------+------+-----+---------+----------------+

mysql> desc sales_table;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| sales_id | int | NO | PRI | NULL | |
| prod_id | varchar(45) | YES | | NULL | |
| cust_id | varchar(45) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+


mysql> alter table sales_table add foreign key (cust_id) references cust_table(cust_id);
ERROR 3780 (HY000): Referencing column 'cust_id' and referenced column 'cust_id' in foreign key constraint 'sales_table_ibfk_1' are incompatible.