[Samba] Problem adding sambaGroupMapping

[stefano malini]

Hi,

i show the command-output trying to add the objectClass 
sambaGroupMapping to some entries that have posixGroup objectClass:

root at amahoro:~# ldapmodify -x -W -D 'cn=Manager,dc=amahoro,dc=bi'
Enter LDAP Password:
dn: cn=Students,ou=Groups,dc=amahoro,dc=bi
changetype: modify
add: objectClass
objectClass: posixGroup
objectClass: sambaGroupMapping
sambaSID: S-1-5-21-251852451-2940789264-3475694606
sambaGroupType: 5

ldapmodify: wrong attributeType at line 6, entry 
"cn=Students,ou=Groups,dc=amahoro,dc=bi"

I don't know what is wrong, sambaSID is a correct attribute. Do you have 
ideas?

Thanks