error 39 via authorize.net

Hi there — when people try to checkout on the site in question, anytime its in euros instead of the default USD, authorize.net throws this error and declines the transaction:

Authorize.Net Credit Card Payment Failed (Status code 3 (E00027): The transaction was unsuccessful. Error Code: 39 – The supplied currency code is either invalid, not supported, not allowed for this merchant or doesn’t have an exchange rate.)

We’ve updated to the latest version of the plugin and have the debug logs on for transactions, if seeing any of that would help. Do you have any ideas what may be causing this? It used to work and I don’t believe the client would have changed any settings, so not sure what would make this start happening out of nowhere.