If a domain has subdomains but no resource records, PowerDNS (and, BTW, djdns), replies that the domain does not exist, which is wrong.
foobar.emptynonterminal.example.org. IN A 192.0.2.1
A query for 'ANY foobar.emptynonterminal.example.org' works, a query for ANY emptynonterminal.example.org' says NXDOMAIN which is bad.
For details, see RFC 2136.
Fixed with the 'Empty Non Terminal' (ENT) code.