trouble with the postmaster

I'm having trouble starting the postmaster on ports other then the
default port with the -i option. It starts fine and everything runs well
if I type:
postmaster -i
And if I type:
postmaster -p 1234
it also works
but if I try:
postmaster -i -p 42

I get the error:
FATAL: StreamServerPort: bind() failed: Permission denied
Is another postmaster already running on that port?
If not wait a few seconds and retry.
/usr/local/pgsql/bin/postmaster: cannot create INET stream port

Can anyone give me any insight into my problem?

Thanks,
Ben