Nginx – How to prevent nginx caching blank pages

I have nginx 1.4.3 running on a Ubuntu 12.04 machine. I have nginx set up to cache pages (they are database-driven but stay fairly static). I'm using MySQL and PHP-FPM.

However, I found that I would intermittently get blank pages cached. There were no errors of any kind, and as soon as I delete the appropriate file from /var/cache/nginx the page would come back.

After some investigation, I have found the issue is that if a HEAD request is received, nginx caches a blank response as the full response for that URL. So HEAD /example stores a blank file in the cache file for the /example page, and a subsequent GET /example returns a blank page. (I seem to get HEAD requests regularly from various search engines and bots.)

Here is the relevant site config:

location ~ \.php$ {
  try_files $uri =404;

  fastcgi_cache one;
  fastcgi_cache_key $scheme$host$request_uri;
  fastcgi_cache_valid  200 5m;
  fastcgi_cache_valid  301 302 304 12h;

  include /etc/nginx/fastcgi_params;
  fastcgi_pass 127.0.0.1:9000;
  fastcgi_index index.php;
  fastcgi_param SCRIPT_FILENAME /srv/www/mysite/public$fastcgi_script_name;
  fastcgi_param HTTPS off;
}

Is this a known bug in nginx? I haven't been able to find any information on this though various searches.

Is there a workaround? There is no way to prevent caching HEAD requests according to this.

I thought maybe there is some 'request method' variable that could be added to fastcgi_cache_key, so that HEAD and GET requests are cached separately. But I cannot find anything.