Container class not appearing on navigation menu

I want to render a menu with a container like this:

<nav class="nav">

However, the container is just rendering with the default class name applied which I don’t want.

<nav class="menu-main-nav-container">

I can’t understand where I’m going wrong. Here’s my code:

if ( has_nav_menu( 'nav-menu' ) ) { 
wp_nav_menu( array( 
'theme_location' => 'nav-menu', 
'container' => 'nav', 
'container-class' => 'nav', 
'items_wrap' => '<ul id="%1$s" class="clearfix %2$s">%3$s</ul>' ) );
} 

Anyone able to help?