Nickserver returns 502 Bad Gateway for address at non-existing domain

--- ~ » curl -k -i --silent --max-time 10 -X GET -d address=foo@exmple.org https://unstable.bitmask.net:6425
HTTP/1.1 502 Bad Gateway
Date: Tue, 06 Dec 2016 14:58:38 GMT
Server: Apache
Content-Length: 69
X-Frame-Options: DENY

{"error":"failed to connect: getaddrinfo: Name or service not known"}%  

But https://pgp.mit.edu/pks/lookup?search=foo%40exmple.org&op=index shows that the key simply doesn't exist, so a 404 not found should be the response.

(from redmine: created on 2016-12-06, closed on 2016-12-06, relates #8648 (closed))