That was it … I overlooked the app name. I have one more problem now, as the error below shows. I use the app mostly to serve json, but there are a few barebones html pages served which use assets in here:
$ tree -L 2 apps/koko_web/priv
apps/koko_web/priv
├── gettext
│ ├── en
│ └── errors.pot
└── static
├── css
└── js
It looks like Heroku is looking for an assets folder.
-----> Building dependencies
remote: Installing and caching node modules
remote: /app/tmp/buildpacks/abc8fb9e8be131ec2574c4ba9e31b81540d97b3bbce47d96e05959c4f81404ac71605dd35ebbcf7a1abe958c346d8ee266ff035de9254424eb54e8b9480059be/lib/build.sh: line 105: cd: /tmp/build_5ac443444d563fb39531eea19bf25c13/./assets: No such file or directory