diff --git a/lib/compat/wordpress-5.9/class-wp-theme-json-5-9.php b/lib/compat/wordpress-5.9/class-wp-theme-json-5-9.php
index 19dd2545a816c1..05f32d0d6a689d 100644
--- a/lib/compat/wordpress-5.9/class-wp-theme-json-5-9.php
+++ b/lib/compat/wordpress-5.9/class-wp-theme-json-5-9.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * WP_Theme_JSON_Gutenberg class
+ * WP_Theme_JSON_5_9 class
  *
  * @package gutenberg
  */
@@ -21,7 +21,7 @@ class WP_Theme_JSON_5_9 {
 	 *
 	 * @var array
 	 */
-	private $theme_json = null;
+	protected $theme_json = null;
 
 	/**
 	 * Holds block metadata extracted from block.json
@@ -426,7 +426,7 @@ private static function sanitize( $input, $valid_block_names, $valid_element_nam
 			return $output;
 		}
 
-		$output = array_intersect_key( $input, array_flip( self::VALID_TOP_LEVEL_KEYS ) );
+		$output = array_intersect_key( $input, array_flip( static::VALID_TOP_LEVEL_KEYS ) );
 
 		// Some styles are only meant to be available at the top-level (e.g.: blockGap),
 		// hence, the schema for blocks & elements should not have them.