How to remove unwanted menu title being printed on the page after the menu?

I have a secondary menu in my WordPress-Theme, and it works – the only problem is that at the end of the ul-list it prints the menu-name.

The output is something like this:

<ul>
    <li><a>One</a></li>
    <li><a>Two</a></li>
    <li><a>Three</a></li>
    secondary
</ul>

I can′t find where this “secondary” comes from, as I did not ask WordPress to give me that information ??

In my theme′s functions.php I register my menus like this:

register_nav_menus( array(
    'primary' => 'Primary Menu',
    'secondary' => 'Secondary Menu',
) );

And in my footer I call the menu:

<?php wp_nav_menu( array( 'theme_location' => 'secondary', 'menu_id' => 'secondary-menu' ) ); ?>
It′s just a weird bug somewhere, especially because it doesn′t appear on my primary menu, only on this secondary menu in my footer. – I appreciate every hint or tip on how to fix this.

I′d love to post a url but the site is still in a closed dev-environment.. But if You need any more code I′m happy to post more.

Thanks in advance for tips, hints and suggestions!