Heroku: "No default language could be detected for this app" error thrown for node app

I am learning NodeJS and the course I'm following has several projects, arranged by sections. I have all the projects under one main folder, which is also a git repository.

Each of these subfolders in the main folder is a node project by itself, complete with package.json and related dependencies in node_modules. The problem is when I tried to push the node app in one such folder(todo-api) to heroku, I get the following error -

remote: Compressing source files... done.
remote: Building source:
remote: 
remote:  !     No default language could be detected for this app.
remote:                         HINT: This occurs when Heroku cannot detect the buildpack to use for this application automatically.
remote:                         See https://devcenter.heroku.com/articles/buildpacks
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to agile-forest-32749.
remote: 
To https://git.heroku.com/agile-forest-32749.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/agile-forest-32749.git'

This is the directory structure for the project -

I decided to refactor out the todo-api subfolder into a new project all by itself, and this time the push to Heroku works just fine -

I don't understand why I get the "no default language" error, especially when the node app is exactly the same in both places. Any thoughts?

via Manish