Amplify build fails due to '"appRoot" key needs to be a string'

After merging a new pull request in my GitHub repository, the automatic build of my Amplify App fails due to 'Invalid Monorepo spec provided. The "appRoot" key needs to be a string'.

I have not changed any of the settings of the repository or amplify app including the environment variables. When checking the environment variables and build settings, they both look like this: build settings environment variables.

I have tried reconnecting the repository as well as merging a test pull request to trigger the automatic build, however it still fails.

I was able to fix this issue by changing my yml from:

applications:
  - frontend: ...
    appRoot: frontend
  - backend: ...

to:

applications:
  - appRoot: frontend
    frontend: ...
    backend: ...

where the appRoot: frontend is the root of my monorepo