Invalid positive integer error message

Hi

A user is getting "Invalid positive integer" error message at the point when they enter the card number details and click continue.

See attached file.

They are trying to register multiple people on a single event.

Can't anything in the watchdog or configandlog.

I guess this is Stripe complaining https://help.formstack.com/hc/en-us/articles/360019519071-Stripe-Commonly-Returned-Errors

Thanks Mike