diff --git a/src/Clients/AbstractMindboxClient.php b/src/Clients/AbstractMindboxClient.php
index 23a6b48..9c366a7 100644
--- a/src/Clients/AbstractMindboxClient.php
+++ b/src/Clients/AbstractMindboxClient.php
@@ -88,7 +88,7 @@ public function __construct($secretKey, IHttpClient $httpClient, LoggerInterface
     public function prepareRequest(
         $method,
         $operationName,
-        \Mindbox\DTO\DTO $body = null,
+        ?\Mindbox\DTO\DTO $body = null,
         $additionalUrl = '',
         $queryParams = [],
         $isSync = true,
@@ -211,7 +211,7 @@ abstract protected function prepareAuthorizationHeader();
      *
      * @return string
      */
-    abstract protected function prepareBody(\Mindbox\DTO\DTO $body = null);
+    abstract protected function prepareBody(?\Mindbox\DTO\DTO $body = null);
 
     /**
      * Инициализация экземпляра запроса.
diff --git a/src/Clients/MindboxClientV2.php b/src/Clients/MindboxClientV2.php
index 470d662..7a8dcfc 100644
--- a/src/Clients/MindboxClientV2.php
+++ b/src/Clients/MindboxClientV2.php
@@ -126,7 +126,7 @@ protected function prepareAuthorizationHeader()
      *
      * @return string
      */
-    protected function prepareBody(DTO $body = null)
+    protected function prepareBody(?DTO $body = null)
     {
         return $body ? $body->toXML() : '';
     }
diff --git a/src/Clients/MindboxClientV3.php b/src/Clients/MindboxClientV3.php
index ac55b0c..410d1b3 100644
--- a/src/Clients/MindboxClientV3.php
+++ b/src/Clients/MindboxClientV3.php
@@ -149,7 +149,7 @@ protected function prepareAuthorizationHeader()
      *
      * @return string
      */
-    protected function prepareBody(\Mindbox\DTO\DTO $body = null)
+    protected function prepareBody(?\Mindbox\DTO\DTO $body = null)
     {
         return $body ? $body->toJson() : '';
     }