The 2nd argument passed into ::Sprockets::Manifest should be a filename

[wasik]

::Sprockets::Manifest expects the manifest filename as the 2nd argument, not a directory. ( Source: https://github.com/rails/sprockets/blob/main/lib/sprockets/manifest.rb )

In most cases, these are in the same location, so it doesn't matter. However, specifying .dir instead of .filename causes problems in cases where the sprockets manifest file was stored in a different location than the asset directory, which can be done in rails by setting config.assets.manifest = 'full-path-to-manifest.json'