Is this a bug? If not, why I'm having this error? Any idea?
No, it is by design. You are encountering the error because that's what it is designed to do. Whether or not that is the correct design is a different question. But everything is working here as it is supposed to currently.
--Quanah -- Quanah Gibson-Mount Platform Architect Zimbra, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration