Ldaprecord Extension Problem

Hi everyone, please any one here ever used the ldap extension before? I have problems with searching data using a filter, there's an error and I don't know why. Please has anyone here used the ldaprecord extension before?
Here's the code for my controller

$filter="(|(cn=a*)(givenname=a*))";
$model_ldap = CLdapRecord::model('LdapNameless');
$model_ldap->setBranchDn('ou=People');
$findAll = $model_ldap->findAll($criteria=array('filter'=>$filter));

and it's saying

 Undefined index: sn 

I don't know why!
Please, someone?