Ryanair identity document requirement (document with no expiry)

Passenger is travelling with a Spanish national ID card that has an
expiry date of 01/01/9999 (so effectively doesn't expire). Ryanair
online check-in only has expiry year up to 2029. Phoning Ryanair gets
the response that you should put the expiry date as equal to the date of
the flight (or the date of the return flight). This information isn't
available on their website. Has anyone had to do this and can confirm it
works....