Be sure that admin_group exists before trying to check for user membership.

lib/gitlab/ldap/access.rb

@@ -110,7 +110,9 @@ module Gitlab
 
       def update_admin_status
         admin_group = Gitlab::LDAP::Group.find_by_cn(ldap_config.admin_group, adapter)
-        if admin_group.has_member?(Gitlab::LDAP::Person.find_by_dn(user.ldap_identity.extern_uid, adapter))
+        admin_user = Gitlab::LDAP::Person.find_by_dn(user.ldap_identity.extern_uid, adapter)
+
+        if admin_group && admin_group.has_member?(admin_user)
           unless user.admin?
             user.admin = true
             user.save