diff --git a/code/Model/SiteTree.php b/code/Model/SiteTree.php
index d624bfed..efb665a8 100755
--- a/code/Model/SiteTree.php
+++ b/code/Model/SiteTree.php
@@ -63,6 +63,7 @@ use SilverStripe\Security\PermissionChecker;
 use SilverStripe\Security\PermissionProvider;
 use SilverStripe\Security\Security;
 use SilverStripe\SiteConfig\SiteConfig;
+use SilverStripe\Subsites\Model\Subsite;
 use SilverStripe\Versioned\RecursivePublishable;
 use SilverStripe\Versioned\Versioned;
 use SilverStripe\View\ArrayData;
@@ -70,7 +71,6 @@ use SilverStripe\View\HTML;
 use SilverStripe\View\Parsers\ShortcodeParser;
 use SilverStripe\View\Parsers\URLSegmentFilter;
 use SilverStripe\View\SSViewer;
-use Subsite;
 
 /**
  * Basic data-object representing all pages within the site tree. All page types that live within the hierarchy should
@@ -1740,7 +1740,7 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
      */
     public function DependentPages($includeVirtuals = true)
     {
-        if (class_exists('Subsite')) {
+        if (class_exists(Subsite::class)) {
             $origDisableSubsiteFilter = Subsite::$disable_subsite_filter;
             Subsite::disable_subsite_filter(true);
         }
@@ -1785,7 +1785,7 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
             $items->merge($redirectorList);
         }
 
-        if (class_exists('Subsite')) {
+        if (class_exists(Subsite::class)) {
             Subsite::disable_subsite_filter($origDisableSubsiteFilter);
         }
 
@@ -1804,9 +1804,8 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
         // Disable subsite filter for these pages
         if ($pages instanceof DataList) {
             return $pages->setDataQueryParam('Subsite.filter', false);
-        } else {
-            return $pages;
         }
+        return $pages;
     }
 
     /**
@@ -1875,8 +1874,8 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
                 'AbsoluteLink' => _t(__CLASS__.'.DependtPageColumnURL', 'URL'),
                 'DependentLinkType' => _t(__CLASS__.'.DependtPageColumnLinkType', 'Link type'),
             );
-            if (class_exists('Subsite')) {
-                $dependentColumns['Subsite.Title'] = singleton('Subsite')->i18n_singular_name();
+            if (class_exists(Subsite::class)) {
+                $dependentColumns['Subsite.Title'] = Subsite::singleton()->i18n_singular_name();
             }
 
             $dependentNote = new LiteralField('DependentNote', '<p>' . _t(__CLASS__.'.DEPENDENT_NOTE', 'The following pages depend on this page. This includes virtual pages, redirector pages, and pages with content links.') . '</p>');
diff --git a/tests/php/Model/SiteTreePermissionsTest.php b/tests/php/Model/SiteTreePermissionsTest.php
index db50fe2c..9e595b1a 100644
--- a/tests/php/Model/SiteTreePermissionsTest.php
+++ b/tests/php/Model/SiteTreePermissionsTest.php
@@ -10,6 +10,7 @@ use SilverStripe\Security\Group;
 use SilverStripe\Security\Member;
 use SilverStripe\Security\Security;
 use SilverStripe\SiteConfig\SiteConfig;
+use SilverStripe\Subsites\Extensions\SiteTreeSubsites;
 use SilverStripe\Versioned\Versioned;
 
 /**
@@ -20,11 +21,11 @@ class SiteTreePermissionsTest extends FunctionalTest
 {
     protected static $fixture_file = "SiteTreePermissionsTest.yml";
 
-    protected static $illegal_extensions = array(
-        SiteTree::class => array('SiteTreeSubsites')
-    );
+    protected static $illegal_extensions = [
+        SiteTree::class => [SiteTreeSubsites::class],
+    ];
 
-    public function setUp()
+    protected function setUp()
     {
         parent::setUp();