In case you are not working in Ubuntu, you can check to make sure you have mod_proxy_html enabled in the default configuration file.

1. Find the httpd.conf file (usually you will find it in a folder called conf, for default installation as stated on apache documentation it could be placed in /usr/local/apache2/conf)
2. Inside the httpd.conf file uncomment the line by removing the pound '#' sign from in front of the line (around line 122 for a default file)

LoadModule proxy_html_module modules/mod_proxy_html.so

Then restart apache.