Name servers with no IP addresses are silently ignored
If a name server has a name but no IP address (here, the example uses the existing TLD "foo"), it is silently ignored:
% check-soa -ns "foo d.nic.fr" fr
d.nic.fr.
2001:678:c::1: OK: 2222644718
194.0.9.1: OK: 2222644718
Compare with the case where the domain does not exist;
% check-soa -ns "foobar d.nic.fr" fr
d.nic.fr.
2001:678:c::1: OK: 2222644718
194.0.9.1: OK: 2222644718
foobar.
Cannot get the IPv4 address: NXDOMAIN