The process has been signaled with signal "9"

I added a new file to put my admin routes in, like so in my RouteServiceProvider:

public function map()
{
    $this->mapAdminRoutes();
}

protected function mapAdminRoutes()
    {
        Route::prefix('admin')
            ->name('admin')
            ->middleware('web')
            ->namespace($this->namespace . '\Admin')
            ->group(base_path('routes/admin.php'));
    }

And if I call any controller from that route file, like so:

<?php

Route::get('/', 'AdminController');

The AdminController:

<?php

namespace App\Http\Controllers\Admin;

use App\Http\Controllers\Controller;

class AdminController extends Controller
{
    public function __invoke()
    {
        return view('admin.auth.login');
    }
}

It returns the error: The process has been signaled with signal "9". Not a helpful Ignition error page... can't figure out why.

Any help is appreciated.