Using wp_nav_menu() in admin area seems not to pick up protected pages?

This particular client would like me to add their main menu to the top of all their admin pages, to help them navigation around their site better.

Here's my code, in the theme's functions.php:

function nvpn_admin_header() {
	include (dirname( __FILE__ ) . '/inc/admin_header.php');
add_action('all_admin_notices', 'nvpn_admin_header');

And the admin_header.php include looks like this:

<div class="admin-header">
	<a href="/" class="logo"><img src="<?php echo get_stylesheet_directory_uri(); ?>/img/NVPN_logo.png" alt="Natural Voice Practitioners' Network" width="120"></a>
	<?php wp_nav_menu( array('menu' => 'Main Menu' )); ?>

Problem is, pages which are protected for Members only with Membership 2 Pro are not showing up in this, despite the current user having access to them.

So I'm just wondering if there's anything else I need to load or register which would help me fix this?