diff --git a/src/Api/DomainV4.php b/src/Api/DomainV4.php
index fc2ee3ee..e1d1e70f 100644
--- a/src/Api/DomainV4.php
+++ b/src/Api/DomainV4.php
@@ -99,8 +99,8 @@ public function show(string $domain, array $requestHeaders = [])
      */
     public function create(
         string  $domain,
-        string  $smtpPass = null,
-        string  $spamAction = null,
+        ?string  $smtpPass = null,
+        ?string  $spamAction = null,
         ?bool $wildcard = null,
         ?bool $forceDkimAuthority = null,
         ?array  $ips = null,