Participant user doesn't exists

I have a problem using the Get User API.
First of all i retrieve the list of the participants of a specific meeting.
When i try to retrieve the user via ID, i obtain this error:
“code”: 1001,
“message”: “User does not exist: userId”

How is it possible that the system is returning a user which no exists?
I am using server-to-server Oauth2 authentication.

Hi @nello.nepi

Thanks for reaching out to the Zoom Developer Forum and welcome to our community!
Have you tried passing the email of the user instead of the userID?

Let me know if that helps