diff --git a/includes/transformer/class-transformer-factory.php b/includes/transformer/class-transformer-factory.php
index 65b07da..70c42e4 100644
--- a/includes/transformer/class-transformer-factory.php
+++ b/includes/transformer/class-transformer-factory.php
@@ -278,6 +278,11 @@ class Transformer_Factory {
 				$transformer_mapping = \get_option( 'activitypub_transformer_mapping', self::DEFAULT_TRANSFORMER_MAPPING );
 				$transformer_name = $transformer_mapping[ $post_type ];
 				$transformer_class = $this->get_transformers( $transformer_name );
+
+				if ( null === $transformer_class ) {
+					wp_die("transformer '". $transformer_name . "' not found", '', ["response" => 404]);
+				}
+
 				$transformer_instance = new $transformer_class();
 				$transformer_instance->set_wp_post( $wp_object );
 				return $transformer_instance->transform();