How to configure overlay unique in cn=config

Hello!

I'm trying to configure unique attribue:

unique.ldiff:

dn: olcOverlay=unique,olcDatabase={1}hdb,cn=config objectClass: olcOverlayConfig objectClass: olcUniqueConfig olcOverlay: unique olcUniqueAttribute: mail

ldapadd -v -Y EXTERNAL -H ldapi:/// -f unique.ldiff ldap_initialize( ldapi:///??base ) SASL/EXTERNAL authentication started SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth SASL SSF: 0 add objectClass: olcOverlayConfig olcUniqueConfig add olcOverlay: unique add olcUniqueAttribute: mail adding new entry "olcOverlay=unique,olcDatabase={1}hdb,cn=config" ldap_add: Invalid syntax (21) additional info: objectClass: value #1 invalid per syntax

What I'm doing wrong ?