Encoding conversion problem in AIX 4.3.3?

Hi,
I'm using AIX 4.3.3 and PostgreSQL 7.3.1, configure command is:

'--prefix=XXXX' '--without-readline' '--without-zlib' '--enable-debug'

created a DB with UNICODE encoding scheme, but when I:

bash-2.05$ export PGCLIENTENCODING=GBK
bash-2.05$ psql -l
psql: ERROR: Conversion between GBK and UNICODE is not supported

what's the problem, but on the other AIX box, it's ok to use GBK
encoding in client side. Don't know why, anybody could give a hand?

Thanks!

Laser