What does the "(null)" in "4D1-3.147 IP6 yrsa ((null))" mean?

[Jan B. Andersen]

When I logon (using telnet) to a SUN running SunOS it normally
displays something like:

  SunOS UNIX (hostname)

I'm setting up an IRIS-4D workstation named yrsa (defined in /etc/sys_id)
which shows

  4D1-3.147 IP6 yrsa ((null))

instead. What does this "(null)" mean and how do I change it?
I'm not running YP if that is of any importance.

I have looked in all the manuals, but without success.
-- 
Jan B. Andersen                              ("SIMULA does it with CLASS")