What this error means?

[Athan Dimoy]

In Nginx logs I've noticed an error saying "close() socket 5 failed (22: 
Invalid argument)". It appears a few times only (2-3 last month)
May I ask what's this error about?

I'm running Nginx 5.0.33, PHP fcgi (using unix socket) and FreeBSD 6.1

Athan