Mastercard Reason Code 4812

What is

The card number used doesn't match any account the issuer has on record.

A data-entry error, expired BIN, or fraudulent card-number generation resulted in a charge against a non-existent account.

Usually points to card-testing fraud or a data-entry error rather than a legitimate customer dispute.

Your checkout's card-validation log at time of purchase.

Submit your system's validation log showing the card number was format-valid at checkout.

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Not running basic card-number validation (Luhn check, BIN lookup) before attempting authorization.

Add card-testing detection (velocity limits on failed auths) to your checkout to catch this pattern early.