php artisan [BadMethodCallException] Method fallback does not exist

Unable to execute any php artisan scripts.

Downgrading a laravel 5.8 installation to 5.2, and have only copied across (manually) my routes, views, and controllers.

I'm trying to run composer install which eventually calls the post-update-cmd composer script.

It fails at php artisan optimize and upon further inspection even running php artisan fails with the same error:

$ php artisan optimize

[BadMethodCallException] Method fallback does not exist.

What am I missing?

I have checked that composer has correctly generated the autoloader, here are the results:

$ composer dumpautoload
Generating autoload filesCarbon 1 is deprecated, see how to migrate to Carbon 2.
https://carbon.nesbot.com/docs/#api-carbon-2
    You can run ".\vendor\bin\upgrade-carbon" to get help in updating carbon and other frameworks and libraries that depend on it.
Generated autoload files containing 518 classes

$ php artisan


  [BadMethodCallException]
  Method fallback does not exist.

Thank-you.

via Chebli Mohamed