The composer install command gives illegal offset type error - laravel

Question

When I run the composer install command on my laravel project, I get this error:
> #php artisan package:discover --ansi
ErrorException : Illegal offset type
at /var/www/routes/web.php:57
56| $blah = app('config')->get('app.api_content_type_map');
> 57| $bb = $blah[$lang];
Exception trace:
1 Illuminate\Foundation\Bootstrap\HandleExceptions::handleError()
/var/www/routes/web.php:57
2 Illuminate\Routing\RouteFileRegistrar::{closure}()
/var/www/vendor/laravel/framework/src/Illuminate/Routing/Router.php:414
Please use the argument -v to see more details.
Script #php artisan package:discover --ansi handling the post-autoload-dump event returned with error code 1
I have this in my config/app.php
'api_content_type_map' => [
'en'=>[
'news'=>'article',
'page'=>'page'
],
'fr'=>[
'nouvelles'=>'article',
'page'=>'page'
]
],
What did I do wrong?
UPDATE 1
This is the output of my php -v
PHP 7.4.2 (cli) (built: Jan 23 2020 11:21:30) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
with Zend OPcache v7.4.2, Copyright (c), by Zend Technologies
UPDATE 2
I deleted line 57, php artisan config:clear, add back the line,composer install, and still get the same error message. I went to another computer, did a fresh checkout of the project, tried to runcomposer install`, also the same error. So I'm thinking it's not actually some cache issue?