diff --git a/src/Message.php b/src/Message.php
index be527cff..7f71654e 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -7,6 +7,9 @@
 use DateTimeImmutable;
 use RuntimeException;
 
+/**
+ * @template TId of AggregateRootId
+ */
 final class Message
 {
     public const TIME_OF_RECORDING_FORMAT = 'Y-m-d H:i:s.uO';
@@ -52,6 +55,9 @@ public function aggregateVersion(): int
         return (int) $version;
     }
 
+    /**
+     * @return TId|null
+     */
     public function aggregateRootId(): ?AggregateRootId
     {
         return $this->headers[Header::AGGREGATE_ROOT_ID] ?? null;