500 internal server error when using reverse-proxy

I have apache2 installed on my Ubuntu 14.04 server. In port 80 apache running while in port 9000 nodeJs app is running with forever. So I added to following lines to port 80 virtualhost in the apache2 config file to run nodeapp in port 80 using reverse-proxy. But now when I visit my site with port 80 I get and error of 500

    ProxyPreserveHost On
    ProxyRequests off
    ProxyPass / http://128.199.136.209:9000/
    ProxyPassReverse / http://128.199.136.209:9000/

The app runs well in port 9000. What is the error got this ?

Have you enabled the necessary Apache modules?

Both mod_proxy and mode_proxy_http.