EE load balancing: DNS resolver throws ArgumentError: comparison of Integer with nil failed

https://sentry.gitlab.net/gitlab/gitlabcom/issues/940830/

ArgumentError: comparison of Integer with nil failed
  net/dns/resolver.rb:1165:in `<'
    while buffer.size < len
  net/dns/resolver.rb:1165:in `block (2 levels) in query_tcp'
    while buffer.size < len
  timeout.rb:93:in `block in timeout'
    return yield(sec)
  timeout.rb:33:in `block in catch'
    ::Kernel.catch(exc) {yield exc}
  timeout.rb:33:in `catch'
    ::Kernel.catch(exc) {yield exc}
...
(13 additional frame(s) were not displayed)

ArgumentError: comparison of Integer with nil failed