Vietnam E-Visa

Hello!

I am currently planning a trip to Vietnam and in the process of filling out the E-Visa application. One of the questions is "Have you ever used any other passports to enter into Viet Nam?" 10 years ago, I did go using an old passport that is now expired and that I don't have in my possession. If I answer "yes," a passport number is asked for. There is no red asterisk next to it to denote it's required information, though. What should I do?

Thanks in advance :)