Menu not displaying with wp_nav_menu

Dear people

I am relatively new to WordPress and am following tutorials, but nothing happens. It’s like all those tutorials are false. Of course, this is all my fault, but I can’t seem to find out why. Here’s the thing.

I have a custom theme with just one index.php template. In that index.php is the line:
wp_nav_menu( array( 'theme_location' => 'hoofdmenu', 'menu_class' => 'nav-menu', 'walker' => new Custom_Walker_Nav_Menu ) );

In the custom theme is the file functions.php with the function stated as:

function register_my_menu() {
  register_nav_menu('hoofdmenu',__( 'Hoofdmenu' ));
}
add_action( 'init', 'register_my_menu' );

On the backend the menu appears and I filled it with ‘Home’ and ‘Voor wie’. The menu is attached to theme location “Hoofdmenu”, with the capital H. Also the checkbox “Automatic add pages (? translation from dutch)” is checked.

The menu is not appearing. Nothing does. Not even some empty html-code. What am I doing wrong?

Please help and thank you!

Maarten