IPN settings in Paypal Sandbox


I am trying to test an integration using my c9.io account and the Paypal Sandbox.
When I set the IPN notification URL, I get the following error in Paypal:

You must enter a properly formatted URL into the Notification URL field in order to activate Instant Payment Notification.

If I visit the URL in a browser, it loads, so I know it is valid. Has anyone been able to set up Paypal Sandbox IPN using a c9.io account?