diff --git src/wp-admin/includes/class-wp-screen.php src/wp-admin/includes/class-wp-screen.php
index 7b7290011c..43db2119de 100644
--- src/wp-admin/includes/class-wp-screen.php
+++ src/wp-admin/includes/class-wp-screen.php
@@ -31,7 +31,7 @@ final class WP_Screen {
 	 * For example, for an `$id` of 'edit-post' the base is 'edit'.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var string|null
 	 */
 	public $base;
 
@@ -47,7 +47,7 @@ final class WP_Screen {
 	 * The unique ID of the screen.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var string|null
 	 */
 	public $id;
 
@@ -89,7 +89,7 @@ final class WP_Screen {
 	 * have a `$parent_base` of 'edit'.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var string|null
 	 */
 	public $parent_base;
 
@@ -99,7 +99,7 @@ final class WP_Screen {
 	 * Some `$parent_file` values are 'edit.php?post_type=page', 'edit.php', and 'options-general.php'.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var string|null
 	 */
 	public $parent_file;
 
@@ -176,7 +176,7 @@ final class WP_Screen {
 	 * Stores the result of the public show_screen_options function.
 	 *
 	 * @since 3.3.0
-	 * @var bool
+	 * @var bool|null
 	 */
 	private $_show_screen_options;
 
@@ -184,7 +184,7 @@ final class WP_Screen {
 	 * Stores the 'screen_settings' section of screen options.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var string|null
 	 */
 	private $_screen_settings;