Using an upstream header value as a proxy_cache_key

[Troy Hakala]

Is it possible to have an upstream server set an HTTP header that can be used as the key in proxy_cache_key? This doesn't work:

                proxy_cache_key $upstream_http_x_myheader;

And neither does this:

                set $v $upstream_http_x_myheader;
                proxy_cache_key $v;


Thanks!