nslookup w/ BIND 8.1.2

[FullName]

What can cause an "unspecified error" message when doing an nslookup
using BIND 8.1.2?  O'Reilly's book on DNS and BIND gave what I thought
was an unsatisfactory explanation to this message.  The "unspecified
error" occurred when I did "ls my.domain" on my own domain name.
Doing an nslookup on a machine running BIND 4.9.7 using the same zone
files returns no error message.

Thx in advance,
ACL