Problem with GPSD refclock

[Rich Wales]

Hi.  I'm trying ntpsec-0.9.6+195 2017-01-18T11:59:53Z on an Ubuntu
16.04.1 LTS Server system with a GPS reference clock managed by gpsd.

The following works just fine under ntpd 4.2.8p4 at 1.3265-o:

server  127.127.46.0	mode 1 minpoll 3 maxpoll 6 prefer
fudge   127.127.46.0	stratum 0 refid GPS

But with ntpsec, when I replaced the above configuration lines with the
following line --

refclock gpsd prefer unit 0 mode 1 minpoll 3 maxpoll 6 stratum 0 refid GPS

-- the reference clock seems to be silently ignored (no errors, but the
refclock doesn't show up in "ntpq -c peers", and the server syncs to one
of its peers instead of to the refclock).

Is there something obvious I might be missing here in the configuration?
 What can people recommend I try at this point to help determine what
the problem is?

Rich Wales
richw at richw.org