They actually seem to work fine but you have to set the non-recursive bit.

  $ dig ns io @b.nic.ac +short

  ; <<>> DiG 9.8.3-P1 <<>> ns io @b.nic.ac +short
  ;; global options: +cmd
  ;; connection timed out; no servers could be reached
  $ dig ns io @b.nic.ac +short +norec
  b.ns13.net.
  [... extracted for brevity]
  ns3.icb.co.uk.