Route with variable gives 404

Hi all,

I'm new to CodeIgniter and have a routing issue.
What I try to do is add a variable to the URL to pass it into the method arguments.

I used this example from the CI manual:

PHP Code:

$routes->add('product/(:num)', 'Catalog::productLookupByID/$1'); 


The actual code to match my needs is:

PHP Code:

$routes->get('/backend/profile/', 'backend/Profile::index');
$routes->get('/backend/profile/(:num)', 'backend/Profile::index/$1'); 


The method in the controller is:

PHP Code:

public function index($var = null): void
{
    echo $var;

    //some more stuff
} 


When I run the URL http://localhost:8080/backend/profile/1

I expect it to echo "1".
But instead a 404 is thrown [Controller or its method is not found: \App\Controllers\backend::index]

This controller does exist, because when I remove the 1 from the URL, the page loads, however, it is echoing NULL obviously...

What is this beginner missing here???