POS errors only with Monzo in Unimini lunch shop

I just got lunch from a place near Monument in London and they had a sign up saying they don’t accept Monzo and I asked why and apparently they get errors on their POS machine, though they do eventually get the payment but they have to ask to check people’s Monzo apps because they can’t tell whether the payment has been a success through their POS.
They were ok for me to pay, I even tried via Google Pay to see if it made a difference, but they still got an error, which I took a picture of, although apparently it’s not always the same error.

Am happy for a Monzonaut to rummage in my account and look at the transaction if that helps, or I can ask for more details from the shop - model and version numbers of the POS machine if that helps?


It’s because Monzo use letters in their authorisation code returned when transactions are being run through the card processor.

Whilst it is allowed, no major bank does this. They all return numeric codes. Monzo, however, returns ones with letters in which some systems then reject.

There’s a number of places that are running their payment systems on old standards and it causes this issue. 3 Stores also have (had?) this issue for new Pay Monthly customers in my personal experience.

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Hi @vitaminj :wave:

So sorry about the problem here!

Would you be able to DM me your email address and I’ll have a look into this :eyes:


We fixed Three by returning numeric codes for them. :slightly_smiling_face:

That’s really interesting.

What’s the motivation for not using numeric codes for everyone? For most merchants I imagine the letters aren’t an issue it’s just the few outliers.

Does the user-agent (or similar) of the POS terminal make its way all the way through to you so you can apply code fixes just for dubious systems (if this is indeed the issue here)?

Codes with letters as well as numbers provide less chance for auth code collisions when it comes time to match authorisations to presentments. It’s also the Mastercard recommended way of doing auth codes now so it won’t just be Monzo cards failing here.

Sadly not, we’ve actually never seen it be a POS issue before, it’s always been something outside of the payments loop (such as bad validation on a web form as with Three).

We’ll get in contact with everybody involved and someone in my team will work out how best to tackle it if we can’t get them to fix it.


I came here to mention the same restaurant near Monument.

Despatch the MonzoNinjas to the venue to resolve.

Although the food is awesome!!!

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