Re-ordering the Admin Menu – made an attempt

Hello,

I’ve made an attempt to re-order the Admin menu for non-Admin users.

It is working when I login as Editor or below and looks great, no errors. But, I’m getting a strange error when I login as Admin.

Code I am using (9 lines):

function menu_order_filter($menu) {
	if(!current_user_can('add_users')) {
		$dev_menu = array('edit.php?post_type=pet','separator1', 'edit.php', 'edit.php?post_type=page', 'separator2');
		array_splice($menu, 1, 0, $dev_menu);
		return array_unique($menu);
	}
}
add_filter('custom_menu_order', create_function('', 'return true;'));
add_filter('menu_order', 'menu_order_filter');

Error when logged in as Admin:
Warning: array_flip() expects parameter 1 to be array, null given in C:\xampp\htdocs\wordpress\wp-admin\includes\menu.php on line 193

The error doesn’t prevent WordPress from loading or functioning. It just appears at the top of the Dashboard when logged in as “admin”, as a line of text at the top.

I’m not an experienced coder so expect I’ve done something basic wrong. Would appreciate some advice!