Nginx returns 204 instead 200

I have simple erlang web application with cowboy web server. I'm trying to launch it behind nginx reverse proxy.

My /etc/nginx/sites-available/default:

server {
        listen 9090;

        default_type text/html;

        location /my_app {
          proxy_pass http://localhost:8001/;
        }
}

If i'm openning http://localhost:9090/my_app it's ok, i'm seeing my html page from my web application. But if i'm adding Content-Type: text/html header to the cowboy response i'm getting 204 instead 200.

Thank you.