diff --git a/src/FcmMessage.php b/src/FcmMessage.php
index bb68ff0..f5298db 100644
--- a/src/FcmMessage.php
+++ b/src/FcmMessage.php
@@ -94,7 +94,7 @@ public function getData(): ?array
     }
 
     /**
-     * @param  array|null  $data
+     * @param  array<string, string>|null  $data
      * @return FcmMessage
      */
     public function setData(?array $data): self