diff --git a/src/Prometheus/Storage/Redis.php b/src/Prometheus/Storage/Redis.php
index bc7acbc0..686c170a 100644
--- a/src/Prometheus/Storage/Redis.php
+++ b/src/Prometheus/Storage/Redis.php
@@ -142,7 +142,7 @@ public function wipeStorage(): void
     /**
      * @inheritDoc
      */
-    public function wipeKey(string $type, string $key)
+    public function wipeKey(string $type, string $key): void
     {
         $this->ensureOpenConnection();