diff --git a/includes/activity/class-event.php b/includes/activity/class-event.php
index f80f6d2..658f3a6 100644
--- a/includes/activity/class-event.php
+++ b/includes/activity/class-event.php
@@ -93,6 +93,12 @@ class Event extends Base_Object {
 	 */
 	protected $status;
 
+	/**
+	 * TODO this seems to not be a default property of an Object but needed by mobilizon TODO:
+	 */
+	protected $actor;
+
+
 	/**
 	 * @context https://joinmobilizon.org/ns#externalParticipationUrl
 	 * @var string
diff --git a/includes/transformer/class-factory.php b/includes/transformer/class-factory.php
index 727e239..beee0b0 100644
--- a/includes/transformer/class-factory.php
+++ b/includes/transformer/class-factory.php
@@ -10,9 +10,12 @@ use Activitypub\Transformer\Attachment;
  */
 class Factory {
 	const DEFAULT_TRANSFORMER_MAPPING = array(
-		'post' => 'activitypub/post',
-		'page' => 'activitypub/post',
-		'attachment' => 'activitypub/attachment',
+		'WP_Post' => array(
+			'post' => 'activitypub/post',
+			'page' => 'activitypub/post',
+			'attachment' => 'activitypub/attachment',
+		),
+		'WP_Comment' => 'activitypub/comment'
 	);
 
 	/**
@@ -252,7 +255,7 @@ class Factory {
 			return $transformer;
 		}
 
-		// use default transformer
+		// Use the built-in transformers.
 		switch ( get_class( $object ) ) {
 			case 'WP_Post':
 				if ( 'attachment' === $object->post_type ) {
@@ -283,13 +286,12 @@ class Factory {
 			case 'WP_Post':
 				$post_type = get_post_type( $object );
 				$transformer_mapping = \get_option( 'activitypub_transformer_mapping', self::DEFAULT_TRANSFORMER_MAPPING );
-				$transformer_name = $transformer_mapping[ $post_type ];
+				$transformer_name = $transformer_mapping[ 'WP_Post' ][ $post_type ];
 				if ( $transformer_name ) {
 					$transformer_class =  $this->get_transformer_class( $transformer_name );
 				} else {
 					return self::get_default_transformer( $object );
 				}
-	
 				if ( $transformer_class ) {
 					return new $transformer_class( $object );
 				} else {