diff --git a/src/Banking/Statement.php b/src/Banking/Statement.php
index d4643f6..0a9e63f 100644
--- a/src/Banking/Statement.php
+++ b/src/Banking/Statement.php
@@ -23,7 +23,7 @@ class Statement implements \JsonSerializable
     /**
      * @return array
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return get_object_vars($this);
     }
diff --git a/src/Banking/Transaction.php b/src/Banking/Transaction.php
index bea9951..dd97b23 100644
--- a/src/Banking/Transaction.php
+++ b/src/Banking/Transaction.php
@@ -26,7 +26,7 @@ class Transaction implements \JsonSerializable
     /**
      * @return array
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return get_object_vars($this);
     }