使用go-imap包从dovecot IMAP服务器获取电子邮件时出现问题

I'm trying to fetch emails from dovecot server using go-imap client implementation. When I use fetch command I get a reply stating BAD Error in IMAP command FETCH: Invalid messageset

I'm only interested in the subject of email, and I use this code snippet for getting emails:

    c, _:= client.DialTLS(mailServer, &tls.Config{InsecureSkipVerify: true})    
    seqset := new(imap.SeqSet)
    seqset.AddRange(index, index)

    messages := make(chan *imap.Message, 10)
    done = make(chan error, 1)
    go func() {
        done <- c.Fetch(seqset, []imap.FetchItem{imap.FetchEnvelope}, messages)
    }()

    if err := <-done; err != nil {
        panic(err)
    }

It's supposed to get an email from mail server by sequence number, but it isn't functioning well.