diff --git a/ipalib/plugins/baseldap.py b/ipalib/plugins/baseldap.py
index b841435fd..6a37995c5 100644
--- a/ipalib/plugins/baseldap.py
+++ b/ipalib/plugins/baseldap.py
@@ -635,7 +635,7 @@ class LDAPObject(Object):
             objectclasses = config.get(
                 self.object_class_config, objectclasses
             )
-        objectclasses += self.possible_objectclasses
+        objectclasses = objectclasses + self.possible_objectclasses
         # Get list of available attributes for this object for use
         # in the ACI UI.
         attrs = self.api.Backend.ldap2.schema.attribute_types(objectclasses)