Samba and smartcards, no userCertificate

I'm trying to add a user with smartcard to samba4. That is, the user shall use smartcard/pin for login instead of username/password. This is accomplished by (among lots of other things) adding the users certificate to samba db. Parameter to add is 'userCertificate;binary'

However, I cannot add the certificate parameter due to that "userCertificate does not exists in the schema" according to error message.

I use ldapmodify basically as this example https://docs.oracle.com/cd/E19424-01/820-4809/6ng8g5576/index.html

The thing is, when I create a user in the system, he/she always get some objectClass parameters which I haven't asked for. For instance 'Person', 'organizationalPerson' etc. These objecClasses does not have 'userCertificate' as possible parameter. But objectClass 'inetOrgPerson' has 'userCertificate'. So I added that class ('inetOrgPerson') to my user, but no luck. Still 'userCertificate does not exists in the schema'

What to do?

Environment; server: debian, client: debian