diff --git a/core/lib/Drupal/Core/Datetime/Element/Datetime.php b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
index a68927a..735dcc1 100644
--- a/core/lib/Drupal/Core/Datetime/Element/Datetime.php
+++ b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
@@ -100,7 +100,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       );
     }
     else {
-      $date = $element['#default_value'];
+      $date = isset($element['#default_value']) ? $element['#default_value'] : null;
       if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
         $input = array(
           'date'   => $date->format($element['#date_date_format']),