I can't find documention anywhere on what it means when nginx shows 001 as the value of $status in the access_log.

I currently use nginx as a reverse proxy and I get this error when uploading large files (2+ MB though my client_max_body_size is 4 MB) .

Also worth noting, the follow values per my log files:

$upstream_status: -
$upstream_response_time: -
$request_completion:

Right now, the nginx server sits behind a load balancer, so it goes: Request -> Load Balancer -> Nginx -> Origin.

Anyone have any ideas what the 001 code is supposed to represent?