diff --git a/src/Handler.php b/src/Handler.php
index dbc6064..f94af0a 100644
--- a/src/Handler.php
+++ b/src/Handler.php
@@ -13,7 +13,7 @@ abstract class Handler {
     
     public bool $registerFatalErrorHandler = true;
 
-    protected Closure $callback;
+    protected ? Closure $callback;
 
     protected Transport\TransportInterface $transport;
    
diff --git a/src/Util/DumpHandler.php b/src/Util/DumpHandler.php
index f72826f..6ea906f 100644
--- a/src/Util/DumpHandler.php
+++ b/src/Util/DumpHandler.php
@@ -6,7 +6,7 @@ use Negundo\Client\DataInterface;
 
 class DumpHandler {
 
-    public DataInterface $dataManipulator;
+    public ? DataInterface $dataManipulator;
 
     public function __construct(DataInterface $dataManipulator = null)
     {
diff --git a/src/Util/TaskHandler.php b/src/Util/TaskHandler.php
index c39d2a4..939e598 100644
--- a/src/Util/TaskHandler.php
+++ b/src/Util/TaskHandler.php
@@ -6,7 +6,7 @@ use Negundo\Client\DataInterface;
 
 class TaskHandler {
 
-    public DataInterface $dataManipulator;
+    public ? DataInterface $dataManipulator;
 
     public function __construct(DataInterface $dataManipulator = null)
     {