Problems with nginx and fastcgi

I am trying to test my server with ab… and getting this kind of error
in the logs:

2008/11/07 21:48:07 [error] 5556#0: *138 upstream prematurely closed
connection while reading response header from upstream, client:
84.242.66.126, server: www.example.org, request: “GET /uptime.php
HTTP/1.0”, upstream: “fastcgi://127.0.0.1:50000”, host:
www.example.org
2008/11/07 21:48:15 [error] 5556#0: *142 upstream prematurely closed
connection while reading response header from upstream, client:
84.242.66.126, server: www.example.org, request: “GET /uptime.php
HTTP/1.0”, upstream: “fastcgi://127.0.0.1:50000”, host:
www.example.org
2008/11/07 21:48:15 [error] 5556#0: *141 upstream prematurely closed
connection while reading response header from upstream, client:
84.242.66.126, server: www.example.org, request: “GET /uptime.php
HTTP/1.0”, upstream: “fastcgi://127.0.0.1:50000”, host:
www.example.org

Does anybody know the issue?