diff --git a/src/ExecutionTrait.php b/src/ExecutionTrait.php
index 27484b2..7fd3deb 100644
--- a/src/ExecutionTrait.php
+++ b/src/ExecutionTrait.php
@@ -263,7 +263,7 @@ public static function checkExecutingUser(array $allowedUsers = []): void
      *
      * @throws \Exception
      */
-    public static function executeChildProcesses(MonitoringItem $monitoringItem, array $workload, int $numberOfchildProcesses = 5, int $batchSize = 10, callable $callback = null, $startAfterPackage = null): void
+    public static function executeChildProcesses(MonitoringItem $monitoringItem, array $workload, int $numberOfchildProcesses = 5, int $batchSize = 10, ?callable $callback = null, $startAfterPackage = null): void
     {
         $workloadChunks = array_chunk($workload, $batchSize); //entries per process
         $childProcesses = $monitoringItem->getChildProcesses();