diff --git a/src/Extensions/FileSubsites.php b/src/Extensions/FileSubsites.php
index 616a2fb1..d389ff89 100644
--- a/src/Extensions/FileSubsites.php
+++ b/src/Extensions/FileSubsites.php
@@ -49,7 +49,7 @@ public function alternateTreeTitle()
      * @param SQLSelect $query
      * @param DataQuery|null $dataQuery
      */
-    protected function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
+    protected function augmentSQL(SQLSelect $query, ?DataQuery $dataQuery = null)
     {
         if (Subsite::$disable_subsite_filter) {
             return;
diff --git a/src/Extensions/GroupSubsites.php b/src/Extensions/GroupSubsites.php
index d66f8b6e..7e6d2707 100644
--- a/src/Extensions/GroupSubsites.php
+++ b/src/Extensions/GroupSubsites.php
@@ -148,7 +148,7 @@ protected function updateTreeTitle(&$title)
      * @param SQLSelect $query
      * @param DataQuery|null $dataQuery
      */
-    protected function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
+    protected function augmentSQL(SQLSelect $query, ?DataQuery $dataQuery = null)
     {
         if (Subsite::$disable_subsite_filter) {
             return;
diff --git a/src/Extensions/LeftAndMainSubsites.php b/src/Extensions/LeftAndMainSubsites.php
index ae2e4fa7..a42cd8cc 100644
--- a/src/Extensions/LeftAndMainSubsites.php
+++ b/src/Extensions/LeftAndMainSubsites.php
@@ -228,7 +228,7 @@ public function shouldChangeSubsite($adminClass, $recordSubsiteID, $currentSubsi
      *
      * @param Member $member
      */
-    protected function canAccess(Member $member = null)
+    protected function canAccess(?Member $member = null)
     {
         if (!$member) {
             $member = Security::getCurrentUser();
@@ -256,7 +256,7 @@ protected function canAccess(Member $member = null)
      *
      * @param Member $member
      */
-    public function alternateAccessCheck(Member $member = null)
+    public function alternateAccessCheck(?Member $member = null)
     {
         return $this->canAccess($member);
     }
diff --git a/src/Extensions/SiteConfigSubsites.php b/src/Extensions/SiteConfigSubsites.php
index 5a17836d..5aec1082 100644
--- a/src/Extensions/SiteConfigSubsites.php
+++ b/src/Extensions/SiteConfigSubsites.php
@@ -29,7 +29,7 @@ class SiteConfigSubsites extends Extension
      * @param SQLSelect $query
      * @param DataQuery|null $dataQuery
      */
-    protected function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
+    protected function augmentSQL(SQLSelect $query, ?DataQuery $dataQuery = null)
     {
         if (Subsite::$disable_subsite_filter) {
             return;
diff --git a/src/Extensions/SiteTreeSubsites.php b/src/Extensions/SiteTreeSubsites.php
index b03f920a..41434963 100644
--- a/src/Extensions/SiteTreeSubsites.php
+++ b/src/Extensions/SiteTreeSubsites.php
@@ -72,7 +72,7 @@ public function isMainSite()
      * @param SQLSelect $query
      * @param DataQuery $dataQuery
      */
-    protected function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
+    protected function augmentSQL(SQLSelect $query, ?DataQuery $dataQuery = null)
     {
         if (Subsite::$disable_subsite_filter) {
             return;