diff --git a/src/bundle/Registry/CronJobsRegistry.php b/src/bundle/Registry/CronJobsRegistry.php
index fdbd1d6..7978fff 100644
--- a/src/bundle/Registry/CronJobsRegistry.php
+++ b/src/bundle/Registry/CronJobsRegistry.php
@@ -44,7 +44,7 @@ public function __construct(string $environment, SiteAccessServiceInterface $sit
         $this->siteAccessService = $siteAccessService;
     }
 
-    public function addCronJob(Command $command, string $schedule = null, string $category = self::DEFAULT_CATEGORY, string $options = ''): void
+    public function addCronJob(Command $command, ?string $schedule = null, string $category = self::DEFAULT_CATEGORY, string $options = ''): void
     {
         $commandName = $command->getName();
         if (null === $commandName) {