nginx not caching response from reverse proxy

http://nginx.org/en/docs/http/ngx_http_memcached_module.html

Basic config is here:

worker_processes  2;

events {
    worker_connections  1024;
}

error_log  /var/log/nginx/nginx_error.log  warn;
error_log  /var/log/nginx/nginx_error.log  info;

http {
    upstream backend {
        server localhost:3000;
    }

  server {
      listen 80;

      location / {
          set            $memcached_key $uri;
          memcached_pass 127.0.0.1:11211;
          error_page     404 = @fallback;
      }

      location @fallback {
          proxy_pass     http://backend;

      }
  }
}

It reverse proxy's the request when hitting port 80, but the logs always say:

2016/08/23 15:25:19 [info] 68964#0: *4 key: "/users/12" was not found by memcached while reading response header from upstream, client: 127.0.0.1, server: , request: "GET /users/12 HTTP/1.1", upstream: "memcached://127.0.0.1:11211", host: "localhost"

Copyright Notice：Content Author:「Justin Thomas」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/39111707/nginx-not-caching-response-from-reverse-proxy