Error "don't supply roman numeral" when trying to merge two actors

I am trying to merge Mark Adams (LXXIII) nm12898119 into Mark Adams (IV) nm0011187 by means of a name correction. They are the same person, and it seems that LXIII was created in error.

But when I do this, I get an error message "Please only enter the name without roman numerals, we will assign roman numerals to the corrected name if necessary."

I've never had this error before when I've been renaming an actor to merge him into an existing entry.