Postmaster startup

Hi,
I decided to tighten up security on the server and start postmaster as
postgres user on startup.
I have a script my_script that is run on startup, which has root
permissions. From this script I want to start postgres script, which is
owned by postgres (postgerSQL superuser) and has 's' bit set so that it
executes with postgres priviledges. When I try to run my_script, I get
an error:
postmaster: real and effective userids must match
what am I doing wrong?
Thanks for your help in advance,

Oleg