diff --git a/Metrics/Stream/DeltaStorage.php b/Metrics/Stream/DeltaStorage.php
index 066d2d9..52c987c 100644
--- a/Metrics/Stream/DeltaStorage.php
+++ b/Metrics/Stream/DeltaStorage.php
@@ -14,7 +14,7 @@
  */
 final class DeltaStorage
 {
-    private Delta $head;
+    private readonly Delta $head;
 
     public function __construct(private readonly AggregationInterface $aggregation)
     {