Heroku Buildpack Multi - no Cedar-supported app detected

I am trying to use PhantomJS with NodeJS in a worker process in Heroku, but when I switched to buildpack-multi, Heroku gives me this when I push:

-----> Fetching custom git buildpack... done
 !     Heroku push rejected, no Cedar-supported app detected

Here is my .buildpack:

http://github.com/heroku/heroku-buildpack-nodejs.git
http://github.com/stomita/heroku-buildpack-phantomjs.git

I also have a package.json in my root directory.