diff --git a/core/lib/Drupal/Core/Field/FieldItemList.php b/core/lib/Drupal/Core/Field/FieldItemList.php
index 56579e4..d2b003e 100644
--- a/core/lib/Drupal/Core/Field/FieldItemList.php
+++ b/core/lib/Drupal/Core/Field/FieldItemList.php
@@ -336,6 +336,9 @@ public function defaultValuesFormValidate(array $element, array &$form, FormStat
     // Extract the submitted value, and validate it.
     $widget = $this->defaultValueWidget($form_state);
     $widget->extractFormValues($this, $element, $form_state);
+    // Force a non-required field definition.
+    // @see self::defaultValueWidget().
+    $this->definition->required = FALSE;
     $violations = $this->validate();
 
     // Assign reported errors to the correct form element.
diff --git a/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php b/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
index 4d62378..57b02ec 100644
--- a/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
+++ b/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
@@ -95,11 +95,18 @@ public function testFieldAdminHandler() {
 
     // Third step: confirm.
     $this->drupalPostForm(NULL, array(
+      'field[required]' => '1',
       'field[settings][handler_settings][target_bundles][' . key($bundles) . ']' => key($bundles),
     ), t('Save settings'));
 
     // Check that the field appears in the overview form.
     $this->assertFieldByXPath('//table[@id="field-overview"]//tr[@id="field-test"]/td[1]', 'Test label', 'Field was created and appears in the overview page.');
+
+    // Check that the field settings form can be submitted again, even though
+    // though the field is required.
+    // The first 'Edit' link is for the Body field.
+    $this->clickLink(t('Edit'), 1);
+    $this->drupalPostForm(NULL, array(), t('Save settings'));
   }