Consider this pubspec manifest:

name: test
dependencies:
  flutter:
    sdk: flutter
flutter:
  deferred-components:
    - name: blah
      assets: blah

Notice that, under the first and only deferred component, assets has a string value rather than a list.
In this case, the tool will print an error: Expected "assets" key in the 0 element of "deferred-components" to be a list, but got blah (String). However, the value is not a list, so the error is inaccurate.

Interestingly, providing assets: blah directly under flutter results in more accurate error text: Expected "assets" to be a list, but got blah (String).\n This is a result of us using completely separate parsing and validation code for parsing the top-level assets section and the assets sections under any deferred components.