diff --git a/src/Endpoint/ContainerAttachWebsocket.php b/src/Endpoint/ContainerAttachWebsocket.php
index 6009f415..ee6fe023 100644
--- a/src/Endpoint/ContainerAttachWebsocket.php
+++ b/src/Endpoint/ContainerAttachWebsocket.php
@@ -27,7 +27,8 @@ public function getExtraHeaders(): array
         );
     }
 
-    protected function transformResponseBody(ResponseInterface $response, SerializerInterface $serializer, string $contentType = null)
+    protected function transformResponseBody(ResponseInterface $response, SerializerInterface $serializer, ?string
+    $contentType = null)
     {
         if (200 === $response->getStatusCode() && DockerRawStream::HEADER === $contentType) {
             return new AttachWebsocketStream($response->getBody());
diff --git a/src/Endpoint/ContainerLogs.php b/src/Endpoint/ContainerLogs.php
index 573ae648..996b09c3 100644
--- a/src/Endpoint/ContainerLogs.php
+++ b/src/Endpoint/ContainerLogs.php
@@ -11,7 +11,7 @@
 
 class ContainerLogs extends BaseEndpoint
 {
-    protected function transformResponseBody(ResponseInterface $response, SerializerInterface $serializer, string $contentType = null)
+    protected function transformResponseBody(ResponseInterface $response, SerializerInterface $serializer, ?string $contentType = null)
     {
         if (200 === $response->getStatusCode() && DockerRawStream::HEADER === $contentType) {
             return new DockerRawStream($response->getBody());