diff --git a/src/GridFieldConfigurablePaginator.php b/src/GridFieldConfigurablePaginator.php
index 51a037c..298a4b3 100644
--- a/src/GridFieldConfigurablePaginator.php
+++ b/src/GridFieldConfigurablePaginator.php
@@ -424,7 +424,7 @@ public function getActions($gridField)
      * @param  GridField $gridField Not used, but present for parent method compatibility
      * @return GridState_Data
      */
-    protected function getGridPagerState(GridField $gridField = null)
+    protected function getGridPagerState(?GridField $gridField = null)
     {
         if (!$this->gridFieldState) {
             $state = $this->getGridField()->State->GridFieldConfigurablePaginator;