create objectClass in ldap using ldif file

Question

Hi I am trying to create a new objectClass in ldap:

dn: cn=subschemasubentry
changetype: modify
add: objectClasses
objectClasses: ( 1.2.3
 NAME 'group' 
 DESC 'Group'
 SUP ( top ) STRUCTURAL
 MUST ( cn )
 MAY ( name $ description $ groupType $ mail $ targetAddress $ displayName $ sAMAccountName $ managedBy ) )

but the command: docker exec people-openldap-container ldapmodify -x -D "cn=admin,dc=example,dc=org" -w <pwd> -f /etc/input/object_class_group.ldif gives me this error:

ldap_modify: Invalid syntax (21)
    additional info: objectClasses: value #0 invalid per syntax
modifying entry "cn=subschemasubentry"

Any hint what could be the error? The error message is unfortunately no so informative to point out the exact error.

thx