diff --git a/CHANGELOG.md b/CHANGELOG.md
index b1f6405..7a9a38f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Fixed
+
+* Fixed that transformer hook function might also return `null`.
+
 ## [0.2.1] - 2024-11-16
 
 ### Added
diff --git a/README.md b/README.md
index 023e5c2..2e12c7a 100644
--- a/README.md
+++ b/README.md
@@ -100,6 +100,12 @@ We're always interested in your feedback. Feel free to reach out to us via [E-Ma
 
 ## Changelog ##
 
+## Unreleased
+
+### Fixed
+
+* Fixed that transformer hook function might also return `null`.
+
 ### [0.2.1] 2024-11-16 ###
 
 * Initial release on https://wordpress.org/
diff --git a/includes/class-setup.php b/includes/class-setup.php
index 2912002..7d1ddb5 100644
--- a/includes/class-setup.php
+++ b/includes/class-setup.php
@@ -252,7 +252,7 @@ class Setup {
 	 *
 	 * @return \Activitypub\Transformer\Base|null
 	 */
-	public function register_activitypub_event_transformer( $transformer, $wp_object, $object_class ): \Activitypub\Transformer\Base {
+	public function register_activitypub_event_transformer( $transformer, $wp_object, $object_class ): ?\Activitypub\Transformer\Base {
 		// If the current WordPress object is not a post (e.g., a WP_Comment), don't change the transformer.
 		if ( 'WP_Post' !== $object_class ) {
 			return $transformer;