diff --git a/README.md b/README.md
index 310d9f4..5c8ea7a 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,21 @@ composer require gorriecoe/silverstripe-dataobjecthistory
 
 ## Requirements
 
-- silverstripe/framework ^4.0
-- symbiote/silverstripe-gridfieldextensions ^3.1
+This module supports Silverstripe CMS 4 and CMS 5.
+
+### CMS 5
+
+Requires the following module versions:
+
+- `silverstripe/framework` ^5.0
+- `symbiote/silverstripe-gridfieldextensions` ^4.0
+
+### CMS 4
+
+Requires the following module versions:
+
+- `silverstripe/framework` ^4.0
+- `symbiote/silverstripe-gridfieldextensions` ^3.1
 
 ## Maintainers
 
diff --git a/composer.json b/composer.json
index e5a6172..0f58a3e 100644
--- a/composer.json
+++ b/composer.json
@@ -16,8 +16,8 @@
   ],
   "homepage": "http://github.com/gorriecoe/silverstripe-dataobjecthistory",
   "require": {
-    "silverstripe/cms": "^4.0",
-    "symbiote/silverstripe-gridfieldextensions": "^3.1"
+    "silverstripe/cms": "^4.0 || ^5.0",
+    "symbiote/silverstripe-gridfieldextensions": "^3.1 || ^4.0"
   },
   "extra": {
     "installer-name": "dataobjecthistory"
diff --git a/src/extensions/DataObjectHistory.php b/src/extensions/DataObjectHistory.php
index 1fe715e..619e251 100644
--- a/src/extensions/DataObjectHistory.php
+++ b/src/extensions/DataObjectHistory.php
@@ -27,10 +27,9 @@
 class DataObjectHistory extends DataExtension
 {
     /**
-     * Update Fields
-     * @return FieldList
+     * @inheritDoc
      */
-    public function updateCMSFields(FieldList $fields)
+    public function updateCMSFields(FieldList $fields): FieldList
     {
         $owner = $this->owner;
         if ($owner->HistoryFields) {
@@ -39,15 +38,16 @@ public function updateCMSFields(FieldList $fields)
                 $owner->HistoryFields
             );
         }
+
         return $fields;
     }
 
-
     /**
-     * Returns the history fields for this dataobject.
-     * @return FieldList
+     * Returns the history fields for this DataObject.
+     *
+     * @return FieldList|null
      */
-    public function getHistoryFields()
+    public function getHistoryFields(): ?FieldList
     {
         $owner = $this->owner;
         if (!$owner->isLatestVersion()) {
@@ -81,23 +81,22 @@ public function getHistoryFields()
                 Versioned::get_all_versions(
                     $owner->ClassName,
                     $owner->ID
-                )
-                ->sort('Version', 'DESC'),
+                )->sort('Version', 'DESC'),
                 $config
-            )
-            ->addExtraClass('grid-field--history')
+            )->addExtraClass('grid-field--history')
         );
     }
 
     /**
-     * @return Member
+     * @return Member|null
      */
-    public function getAuthor()
+    public function getAuthor(): ?Member
     {
         $owner = $this->owner;
         if ($owner->AuthorID) {
             return Member::get()->byId($owner->AuthorID);
         }
+
         return null;
     }
 }
diff --git a/src/forms/GridFieldHistoryButton.php b/src/forms/GridFieldHistoryButton.php
index 9c261e3..fc6cf41 100644
--- a/src/forms/GridFieldHistoryButton.php
+++ b/src/forms/GridFieldHistoryButton.php
@@ -7,6 +7,7 @@
 use SilverStripe\View\SSViewer;
 use SilverStripe\Forms\GridField\GridFieldViewButton;
 use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\FieldType\DBHTMLText;
 
 /**
  * DataObjectHistory
@@ -15,19 +16,24 @@
  */
 class GridFieldHistoryButton extends GridFieldViewButton
 {
+    /*
+     * @inheritDoc
+     */
     public function getColumnContent($field, $record, $col)
     {
         if ($record->isLatestVersion()) {
             return null;
         }
 
-        $data = new ArrayData(array(
-            'Link' => Controller::join_links(
-                $field->Link('item'),
-                $record->ID,
-                'view?VersionID='. $record->Version
-            )
-        ));
+        $data = new ArrayData(
+            [
+                'Link' => Controller::join_links(
+                    $field->Link('item'),
+                    $record->ID,
+                    'view?VersionID=' . $record->Version
+                )
+            ]
+        );
 
         $template = SSViewer::get_templates_by_class(
             $this,
diff --git a/src/forms/HistoryGridFieldItemRequest.php b/src/forms/HistoryGridFieldItemRequest.php
index 34f0fb1..53dcfff 100644
--- a/src/forms/HistoryGridFieldItemRequest.php
+++ b/src/forms/HistoryGridFieldItemRequest.php
@@ -1,4 +1,5 @@
 <?php
+
 namespace gorriecoe\DataObjectHistory\Forms;
 
 use SilverStripe\Forms\FieldList;
@@ -13,6 +14,10 @@
 use SilverStripe\Versioned\VersionedGridFieldItemRequest;
 use SilverStripe\Versioned\Versioned;
 use SilverStripe\View\ArrayData;
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Control\HTTPResponse;
+use SilverStripe\Forms\Form;
+use SilverStripe\ORM\DataObject;
 
 /**
  * DataObjectHistory
@@ -31,9 +36,9 @@ class HistoryGridFieldItemRequest extends VersionedGridFieldItemRequest
     ];
 
     /**
-     * @var int
+     * @var int|null
      */
-    protected $versionID;
+    protected ?int $versionID = null;
 
     public function __construct($gridField, $component, $record, $requestHandler, $popupFormName)
     {
@@ -45,28 +50,36 @@ public function __construct($gridField, $component, $record, $requestHandler, $p
                 );
             }
         }
+
         parent::__construct($gridField, $component, $record, $requestHandler, $popupFormName);
     }
 
+    /*
+     * @inheritDoc
+     */
     public function view($request)
     {
         if (!$this->record->canView()) {
-            $this->httpError(403);
+            return $this->httpError(403);
         }
+
         $controller = $this->getToplevelController();
         $form = $this->ItemEditForm();
+
         $data = ArrayData::create([
             'Backlink' => $controller->Link(),
             'ItemEditForm' => $form
         ]);
+
         $return = $data->renderWith($this->getTemplates());
         if ($request->isAjax()) {
-            return $return;
+            return HTTPResponse::create($return);
         }
+
         return $controller->customise(['Content' => $return]);
     }
 
-    public function ItemEditForm()
+    public function ItemEditForm(): Form
     {
         $form = parent::ItemEditForm();
         $fields = $form->Fields();
@@ -103,11 +116,13 @@ public function ItemEditForm()
             DBField::create_field('HTMLFragment', $message),
             'notice'
         );
+
         $form->setFields($fields);
+
         return $form;
     }
 
-    public function doRollback($data, $form)
+    public function doRollback(array $data, Form $form): HTTPResponse
     {
         $record = $this->record;
 
@@ -118,18 +133,18 @@ public function doRollback($data, $form)
 
         // Save from form data
         $record->doRollbackTo($record->Version);
-        $link = '<a href="' . $this->Link('edit') . '">"'
+        $link = '<a href="' . $this->Link('edit') . '">'
             . htmlspecialchars($record->Title, ENT_QUOTES)
-            . '"</a>';
+            . '</a>';
 
         $message = _t(
             __CLASS__ . '.RolledBack',
             'Rolled back {name} to version {version} {link}',
-            array(
+            [
                 'name' => $record->i18n_singular_name(),
                 'version' => $record->Version,
                 'link' => $link
-            )
+            ]
         );
 
         $form->sessionMessage($message, 'good', ValidationResult::CAST_HTML);
@@ -138,9 +153,11 @@ public function doRollback($data, $form)
         return $controller->redirect($record->CMSEditLink());
     }
 
-    public function getFormActions()
+    public function getFormActions(): FieldList
     {
+        $actions = parent::getFormActions();
         $record = $this->getRecord();
+
         if (!$record || !$record->has_extension(Versioned::class)) {
             return $actions;
         }
@@ -148,7 +165,6 @@ public function getFormActions()
         $this->beforeExtending('updateFormActions', function (FieldList $actions) use ($record) {
             if (!$record->isLatestVersion()) {
                 $actions->removeByName([
-                    'action_doUnpublish',
                     'action_doUnpublish',
                     'action_doDelete',
                     'action_doSave',
@@ -172,7 +188,6 @@ public function getFormActions()
             }
         });
 
-        $actions = parent::getFormActions();
         return $actions;
     }
 }