How to configure overlay unique in cn=config

[Jarek <jarek@poczta.srv.pl>]

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 ?
-- 
Jarek <jarek@poczta.srv.pl>