diff --git a/includes/class-transformers-manager.php b/includes/class-transformers-manager.php
index aabfb0f..dc425e5 100644
--- a/includes/class-transformers-manager.php
+++ b/includes/class-transformers-manager.php
@@ -278,7 +278,7 @@ class Transformers_Manager {
 		switch ( get_class( $object ) ) {
 			case 'WP_Post':
 				$post_type = get_post_type( $object );
-				$transformer_mapping = \get_option( 'activitypub_transformer_mapping', DEFAULT_TRANSFORMER_MAPPING );
+				$transformer_mapping = \get_option( 'activitypub_transformer_mapping', self::DEFAULT_TRANSFORMER_MAPPING );
 				$transformer_name = $transformer_mapping[ $post_type ];
 				$transformer_instance = new ( $this->get_transformers( $transformer_name ) );
 				$transformer_instance->set_wp_post( $object );