According to https://tools.ietf.org/html/rfc6891#section-6.2.3 responder should round the 'UDP Payload Size' up to 512 (at least I think so). However, for values smaller than 12 Unbound returns SERVFAIL.

How to reproduce:

dig +bufsize=0 +edns .

Expected output:

DNS record for '.'

Acctual output:

SERVFAIL