SSI goes to backend

[zepolen]

In the html for the backend app server is: <!--# include
virtual="http://www.domain.com/ssi/base.html" -->

Going to directly to http://www.domain.com/ssi/base.html in a browser
shows the file ok

However if it gets loaded as ssi, the request seems to go to the
backend (which says 404) - instead of being served from nginx.

Why is this?

Nginx config:

        location / {
            ssi on;
            proxy_set_header Host $http_host;
            proxy_pass http://backend;
            proxy_redirect off;
        }
        location /ssi {
            alias /var/www/ssi;
        }