title: Custom Columns Don't Display Anything
body: <p>I have added a new column to a custom post type to display the post's ID. This works for the WordPress core post type, but not for my custom post type. I have tried using the manage_{post_type}_custom_column hook and just applying it to all posts, but neither works.</p>
  
  <p>It DOES add the custom column headers, but I can't populate them with anything at all when viewing the custom post type.</p>
  
  <p><a href="https://i.stack.imgur.com/mVZmr.png" rel="nofollow noreferrer">This is what it looks like when viewing the custom post type</a>
  and
  <a href="https://i.stack.imgur.com/kPnCP.png" rel="nofollow noreferrer">this is what it looks like when viewing a regular core post.</a></p>
  
  <pre><code>//  Add post ID column to use an order ID in all posts view.
  
  add_filter('manage_posts_columns', 'oms_order_id_header');
  add_action('manage_posts_custom_column', 'oms_order_id_column', 10, 2);
  
  function oms_order_id_header($columns) {
      //Remove title column
      //unset($columns['title']);
      //Add new columns
      $columns['order_id_header'] = 'Order ID';
      $columns['customer_header'] = 'Customer';
      $columns['author']          = 'Owner';    
  
      return $columns;
  }
  
  function oms_order_id_column( $column, $post_id ) {
          if ($column == 'order_id_header') {
              echo $post_id;
          }
  }
  </code></pre>