gzip-static option isn't working on default index

I have gzip-static set in a server block in httpd.conf.

If I make the following request:

curl -I -H 'Accept-Encoding: gzip,deflate' http://example.com

the response has no Content-Encoding header and the Content-Length
matches /var/www/htdocs/index.html.

If I make the following request:

curl -I -H 'Accept-Encoding: gzip,deflate' http://example.com/index.html

the response includes Content-Encoding: gzip and the Content-Length
matches /var/www/htdocts/index.html.gz.

Is there some additional configuration required or is this the intended
output for the option?

Thank you,

Paul