php artisan serve not respond of next url

I use laravel 5.6.34. I installed Laravel with the clone github laravel / laravel and composer install. I make a route something like this

Route::get('/', function () {
    return view('welcome');
});

Route::prefix('admin')->group(function () {
    Route::get('/', function () {
        echo 'Hello Admin';
    });
});

I run php artisan serve it work fine in first url localhost:8000, but when I try to open second url localhost:8000/admin it keep loading. Because I can't wait I stop php artisan serve with ctrl + c, but the strange thing happens when I run exit cmd not closed. Can anyone tell what's wrong? And sorry for bad english.

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