23401 is the zip code for Lagos State in Nigeria

My debit card did not work online until I used 23401 as my zip code.  That's like an electronic/credit card; it's labelled Visa, but issued by a Nigerian bank.

Previously, the specialized customer care from the bank had suggested many reasons why my address wasn't working, but yesterday I found out that Lagos indeed has a five-digit zip code or postcode.  Presumably, other cities/states in the country have 234-something-something too, but good luck finding those since the NIPOST utility at nigeriapostcodes.com is down (domain name expired). 

This postal code issue has confused many Nigerian people, and I suffered so much trying to use my very super special card online until I had to beg my sister and my mother and transfer money just before deadline. 
Then I found out this new secret zipcode, and I have just made a purchase online with it, no problem at all. 

Although they couldn't get the zip code right, the customer service (phone number at the back of your card) helped me figure out which of my many possible addresses (work, shack, parents', other job, ...) I had entered.  I will inform them about this blind spot of theirs.  If you have trouble with your own card, phone the number; for once customer service in Nigeria really tried to be helpful. 

Welcome to the late 20th century, brethren. 

ADDITIONAL POSTCODES
Lagos State 23401
Oyo State 23402
FCT (Abuja) 23409
Maybe the last two digits are from the telephone area code?  (See Nigeria local area codes here)