diff --git a/src/Contracts/DriverInterface.php b/src/Contracts/DriverInterface.php
index 8097e4a..a3f44b3 100644
--- a/src/Contracts/DriverInterface.php
+++ b/src/Contracts/DriverInterface.php
@@ -41,7 +41,7 @@ public function find($code, $active = 1);
      *
      * @return int
      */
-    public function update($code, array $attributes, DateTime $timestamp = null);
+    public function update($code, array $attributes, ?DateTime $timestamp = null);
 
     /**
      * Remove given currency from storage.
@@ -49,4 +49,4 @@ public function update($code, array $attributes, DateTime $timestamp = null);
      * @return int
      */
     public function delete($code);
-}
\ No newline at end of file
+}