You need to check for the presence of 'post_type' as an index of $_GET before using it: if ((isset($_GET['post_type']) && $_GET['post_type'] == 'slide') || (isset($post_type) && $post_type == 'slide')) : Also, you should be using the wp_enqueue_style function instead of echoing your stylesheet at 'admin_head' : wp_enqueue_style( 'meteor-slides-admin', plugins_url('/css/meteor-slides-admin.css', __FILE__), array(), '1.0' ); More information on wp_enqueue_style here . More Solution theme development - How can I alter the display of category listings via sidebar.php? in WordPress cache - How to find time last viewed? in WordPress wp admin - Why can't I exclude private posts from this query? in WordPress php - Display All Post Attachments and Assign Class to the Last Image? in WordPress hierarchical - How to make a list that displays the top level subpages, and any subpages or siblings off of the current page in WordPress Reffered: https://wordpress.org/
