Why is master..sysprocesses.hostname is NULL for my programs spid?

I am running into the following situation

Machine 1
Solaris 10/using mutiple Zones
Sybase ASE 15

Machine 2
Solaris 8
Sybase ASE 12.5

I have a program that makes a connection to a Sybase ASE database and
then makes an entry into a table selecting hostname, hostprocess, spid
from master.sysprocesses where spid is equal to @@spid. machine 1
hostname is always NULL for this select and the insert into my table
fails because I require this field. machine 2 hostname is always
populated for the select and this insert works.

Under what circumstances is hostname set to NULL in sysprocesses? Is
there anything I can do to make sure that when my program makes a
connection that the hostname will be populated for that spid?

Thanks