diff --git a/src/Endpoints/RequestsEndpoint.php b/src/Endpoints/RequestsEndpoint.php
index b1fc06b..04b9f4d 100644
--- a/src/Endpoints/RequestsEndpoint.php
+++ b/src/Endpoints/RequestsEndpoint.php
@@ -135,10 +135,10 @@ public function scheduleValidationCall(
         int $id,
         DateTime $dateTime,
         string $timezone = 'Europe/Amsterdam',
-        string $phoneNumber = null,
-        string $extensionNumber = null,
-        string $email = null,
-        string $comments = null,
+        ?string $phoneNumber = null,
+        ?string $extensionNumber = null,
+        ?string $email = null,
+        ?string $comments = null,
         string $action = 'ScheduledCallback',
         string $language = 'en_us'
     ): Base
@@ -248,4 +248,4 @@ public function cancel(int $id, ?string $reason = null): Base
             'reason' => $reason ?? ''
         ]));
     }
-}
\ No newline at end of file
+}