heroku - could not find a javascript runtime

Question

I am getting the following error message in the heroku logs when trying to access the application from the browser:

 Gem Load Error is: Could not find a
 JavaScript runtime. See https://github.com/rails/execjs for a list of available runtimes.

This is being triggered by a couple of gems - ngannotate-rails and coffee-rails.

This is a rails 4.2 application with a angular.js frontend, being deployed to heroku for the first time.

Heroku stack:

$ heroku stack
=== my_app Available Stacks
  heroku-16
* cedar-14

Buildpacks:

$ heroku buildpacks
=== my_app Buildpack URLs
1. https://github.com/prakashmurthy/heroku-buildpack-nodejs
2. heroku/ruby

Both the node.js build and ruby build complete successfully.

PATH seems to be set correctly:

$ heroku run bash
Running bash on ⬢ my_app... up, run.1502 (Free)
~ $ echo $PATH
/app/bin:/app/vendor/bundle/bin:/app/vendor/bundle/ruby/2.3.0/bin:/usr/local/bin:/usr/bin:/bin

Appreciate any help in figuring out this issue.