imapc tries to fetch uid 0

Hi,

This is a capture from an imapc client session (which is triggered by a message, received via LMTP):

• OK [CAPABILITY IMAP4rev1 NAMESPACE] Ready 2 LOGIN "uid" "pass" 2 OK LOGIN succeeded 1 LIST "" ""
• LIST (\Noselect) "/" "" 1 OK LIST completed 3 APPEND "INBOX" {568}

• Ready for 568 octets of data [...] 3 OK APPEND complete 4 EXAMINE "INBOX"

• FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
• OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)] Flags permitted.
• 4 EXISTS
• 0 RECENT
• OK [UIDVALIDITY 1455885804]
• OK [UIDNEXT 5] 4 OK [READ-ONLY] EXAMINE successful 5 UID FETCH 0 (RFC822.SIZE BODY.PEEK[HEADER.FIELDS (Message-ID)]) 5 BAD Illegal syntax: Bad message number 0

Dovecot 2.2.21

I don't see any problems with the conversation otherwise. Is it safe to assume this is a bug in imapc?