nav_menu doesn't work

I’ll explain the problem with just code, because there is not really anything to explain. I have a very simple theme with only an index, header and style file. This is in my header file:

<?php wp_nav_menu( array('container_class' => 'menubar' )); ?>

and this is what comes on the actual website:

<div class="menu">
<ul>
<li class="page_item page-item-2"><a href="https://blabla.com/daa/?page_id=2">Sample Page</a></li>
</ul>
</div>

As you see, the container class keeps ‘menu’ in stead of menubar…
I have tried with different arguments, but none of them work….

[ please don’t bump – its not permitted on these forums ]