initdb memory segment creation error

I am getting this error with initdb while creating shared segment -
"memory segment exceeded available memory or swap space. To reduce the
request size (currently 1785856 bytes), reduce PostgreSQL's shared_buffers
parameter (currently 50)) and/or its max_connections parameter (currently
13)."
Total memory is 256 MB and memory available to user processes is 178 MB.
It's not an issue with shared segment exceeding SHMMAX but value of SHMMAX
is 67108864 and SHMALL is 8192. It is not exactly a Postgresql problem but
what am I missing here?

Thanks,

Fahad