unix_socket_directory

I'm running pg 7.4.6 on fedora core 2.

I'm trying to set unix_socket_directory for some clustering I'm trying
to accomplish, but get an error when I try to set it in postgresql.conf:

FATAL: syntax error in file "postgresql.conf" line 36, near token "/"

It goes away if I comment out the line, and it doesn't look like it's
made wrong to me.

Here's an excerpt from the postgresql.conf file

# - Connection Settings -

tcpip_socket = true
max_connections = 100
# note: increasing max_connections costs about 500 bytes of
shared
# memory per connection slot, in addition to costs from
shared_buffers
# and max_locks_per_transaction.
unix_socket_directory = /home/pgmaster/tmp
#superuser_reserved_connections = 2
port = 5432

The directory I'm trying to use is world writable. Am I missing
something simple here?