I can't add a person as a co-host

[alinasait]

I use zoom everyday so I have lots of regular sessions. I can't add one person as an alternative host/co-host, it says: 'One or more of the people you want to add are alternate hosts or are not members of this account. To continue delete: name' 

what's wrong? the 2nd person has definitely got an account. How to fix this?

thanks in advance