Unable To Alter Data Type

I have an issue I can't figure out. I have the following TABLE:

tysql=# \d customers
Table "public.customers"
Column | Type | Modifiers
--------------+----------------+-----------
cust_id | character(10) | not null
cust_name | character(50) | not null
cust_address | character(50) |
cust_city | character(50) |
cust_state | character(5) |
cust_zip | character(10) |
cust_country | character(50) |
cust_contact | character(50) |
cust_email | character(255) |
Indexes:
"customers_pkey" PRIMARY KEY, btree (cust_id)
Referenced by:
TABLE "orders" CONSTRAINT "fk_orders_customers" FOREIGN KEY
(cust_id) REFERENCES customers(cust_id)

Now I'm attempting to ALTER the field 'cust_zip' TYPE from character
to integer however I'm getting the following error:

tysql=# ALTER TABLE customers ALTER COLUMN cust_zip TYPE bigint;
ERROR: column "cust_zip" cannot be cast to type bigint

I thought perhaps the stored data in the field conflicted with the
data type but I can't see why:

tysql=# SELECT cust_zip FROM customers;
cust_zip
------------
44444
43333
42222
88888
54545
32765
(6 rows)