diff --git a/src/Plugin/Field/FieldWidget/InlineEntityFormBase.php b/src/Plugin/Field/FieldWidget/InlineEntityFormBase.php
index 2d5f77eb27701cdf17419b04364f215c2d7dc468..69869be8e099ea191d6fdbcfcd5c4acda05c0ad9 100644
--- a/src/Plugin/Field/FieldWidget/InlineEntityFormBase.php
+++ b/src/Plugin/Field/FieldWidget/InlineEntityFormBase.php
@@ -424,7 +424,7 @@ abstract class InlineEntityFormBase extends WidgetBase implements ContainerFacto
    * @return array
    *   IEF form element structure.
    */
-  protected function getInlineEntityForm($operation, $bundle, $langcode, $delta, array $parents, EntityInterface $entity = NULL) {
+  protected function getInlineEntityForm($operation, $bundle, $langcode, $delta, array $parents, ?EntityInterface $entity = NULL) {
     $element = [
       '#type' => 'inline_entity_form',
       '#entity_type' => $this->getFieldSetting('target_type'),
diff --git a/tests/modules/inline_entity_form_test/src/IefTestForm.php b/tests/modules/inline_entity_form_test/src/IefTestForm.php
index c598dbb37845d262c129f49c93e8b6a4eca61050..39d212a252d9d72b2741a1fa8ad60b99f04e584c 100644
--- a/tests/modules/inline_entity_form_test/src/IefTestForm.php
+++ b/tests/modules/inline_entity_form_test/src/IefTestForm.php
@@ -21,7 +21,7 @@ class IefTestForm extends FormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, $form_mode = 'default', Node $node = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, $form_mode = 'default', ?Node $node = NULL) {
     $form['inline_entity_form'] = [
       '#type' => 'inline_entity_form',
       '#entity_type' => 'node',