diff --git a/src/Contracts/MailProviderContract.php b/src/Contracts/MailProviderContract.php
index f769434..f592264 100644
--- a/src/Contracts/MailProviderContract.php
+++ b/src/Contracts/MailProviderContract.php
@@ -4,5 +4,5 @@
 
 interface MailProviderContract
 {
-    public function driver(string $driver = null);
+    public function driver(?string $driver = null);
 }