Почему Nginx не подставляет переменную в proxy_pass?

server {
    # ...
    set $s3_server          "s3.eu-central-1.amazonaws.com";
    set $s3_bucket          "wtf";

    location / {
        expires                 30d;

        proxy_set_header        Host $s3_server;

        proxy_set_header        X-Real-IP $remote_addr;
        proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header        X-Forwarded-Proto $scheme;

        proxy_pass $scheme://$s3_server/$s3_bucket/;
    }
   # ...
}