apache redirect from non www to www

I have a website that doesn’t seem to redirect from non-www to www.

My Apache configuration is as follows:

RewriteEngine On
### re-direct to www
RewriteCond %{http_host} !^www.example.com [nc]
RewriteRule ^(.*)$ http://www.example.com/$1 [r=301,nc] 

What am I missing?