[Theme: Purity] Responsive navigation displays blank

My client is using the Purity v2.3 on the latest version of WordPress. For some reason the responsive navigation is not working. All I see is selection menu with one option, which is “Select page:”.

Here is the responsive_select_nav function from functions.php

function responsive_select_nav() {

	$locations = get_nav_menu_locations();
	$menu = wp_get_nav_menu_object( $locations[ 'custom_menu' ] );

	$items = wp_get_nav_menu_items($menu->term_id);
	global $of_option;
	$prefix = "st_";
	if($of_option[$prefix.'translate']){
		$tr_select_page = $of_option[$prefix.'tr_select_page'];
	}else{
		$tr_select_page = __('Select page:', 'spacing');
	}
	echo "<select id='page_id' name='page_id'>";
	echo "<option>".$tr_select_page."</option>";
	   foreach ($items as $list){
			  if($list->menu_item_parent != "0"){
				echo "<option value=".$list->url.">? ?".$list->title."</option>";
			  }
			  else {
				echo "<option value=".$list->url.">".$list->title."</option>";
			  }

	   }
	echo "</select>";
}

And in header.php it’s added like so.

<div class="nav">
                    <ul>
                    	<?php wp_nav_menu(array('menu' => 'custom_menu')); ?>
                    </ul>
                    <?php responsive_select_nav() ?>
            	</div>

What is wrong? Why is it not displaying?