configuration of nginx for phoenix websockets

I'm seeing

socket.js:229 WebSocket connection to 'ws://mywebsite.io/socket/websocket&vsn=2.0.0' failed.

With this nginx configuration and I'm not sure why

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name mywebsite.io;

    location /socket/websocket/ {
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            proxy_pass http://localhost:4000/socket/websocket/;
    }

    location / {
            proxy_pass http://localhost:4000/;
    }
}