diff --git a/src/Controllers/HistoryViewerController.php b/src/Controllers/HistoryViewerController.php
index 8a5280d..0a1e880 100644
--- a/src/Controllers/HistoryViewerController.php
+++ b/src/Controllers/HistoryViewerController.php
@@ -355,7 +355,7 @@ public function getCompareForm(array $context)
         return $form;
     }
 
-    public function versionForm(HTTPRequest $request = null)
+    public function versionForm(?HTTPRequest $request = null)
     {
         if (!$request) {
             $this->jsonError(400);
@@ -373,7 +373,7 @@ public function versionForm(HTTPRequest $request = null)
         }
     }
 
-    public function compareForm(HTTPRequest $request = null)
+    public function compareForm(?HTTPRequest $request = null)
     {
         if (!$request) {
             $this->jsonError(400);
diff --git a/src/Forms/DataObjectVersionFormFactory.php b/src/Forms/DataObjectVersionFormFactory.php
index a98d1b7..e424c96 100644
--- a/src/Forms/DataObjectVersionFormFactory.php
+++ b/src/Forms/DataObjectVersionFormFactory.php
@@ -38,7 +38,7 @@ class DataObjectVersionFormFactory implements FormFactory
         DataObjectVersionFormFactory::TYPE_HISTORY,
     ];
 
-    public function getForm(RequestHandler $controller = null, $name = FormFactory::DEFAULT_NAME, $context = [])
+    public function getForm(?RequestHandler $controller = null, $name = FormFactory::DEFAULT_NAME, $context = [])
     {
         // Validate context
         foreach ($this->getRequiredContext() as $required) {
@@ -91,7 +91,7 @@ public function isReadonlyFormType(array $context)
         return in_array($this->getFormType($context), $this->config()->get('readonly_types') ?? []);
     }
 
-    protected function getFormFields(RequestHandler $controller = null, $name, $context = [])
+    protected function getFormFields(?RequestHandler $controller, $name, $context = [])
     {
         $record = $context['Record'];
         /** @var FieldList $fields */
@@ -157,7 +157,7 @@ protected function removeSelectedRightTitles(FieldList $fields)
         }
     }
 
-    protected function getFormActions(RequestHandler $controller = null, $formName, $context = [])
+    protected function getFormActions(?RequestHandler $controller, $formName, $context = [])
     {
         $actions = FieldList::create();
         $this->invokeWithExtensions('updateFormActions', $actions, $controller, $formName, $context);