Demokrit on "Class usage in register_sidebar"

Greetings,

I'm trying to customize the markup of the widgets in my sidebar for Bootstrap 3.2.0. According to Codex, "register_sidebar" should support a "class" argument:

register_sidebar(array(
		'name' => __('Primary Sidebar', 'bootstrapped'),
		'id' => 'sidebar-1',
		'description' => __('Main sidebar that appears on the left.', 'bootstrapped'),
		'class' => 'my-class',
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget' => '</aside>',
		'before_title' => '<div class="widget-title">',
		'after_title' => '</div>',
	));

But "class" seems to be ignored (WP 3.9.2./ WP 4.0). Can anyone share a light on it's usage?

I know that I could solve this by css or js, but then my otherwise solid solution would get hacky.