diff --git a/src/Admin/AdminApiInterface.php b/src/Admin/AdminApiInterface.php
index 72e34ff9e..0ee9b242a 100644
--- a/src/Admin/AdminApiInterface.php
+++ b/src/Admin/AdminApiInterface.php
@@ -373,7 +373,7 @@ function checkDirectory(array $paths = []): ?Message\CheckDirectoryResponse;
      * @return Message\CheckDomainMXRecordResponse
      */
     function checkDomainMXRecord(
-        DomainSelector $domain = null
+        ?DomainSelector $domain = null
     ): ?Message\CheckDomainMXRecordResponse;
 
     /**