.../serialization/src/Normalizer/DateTimeIso8601Normalizer.php       | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index 30bcde3514..44e2751d67 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -37,6 +37,7 @@ class DateTimeIso8601Normalizer extends DateTimeNormalizer {
    * {@inheritdoc}
    */
   public function normalize($datetime, $format = NULL, array $context = []) {
+    assert($datetime instanceof DateTimeIso8601);
     $field_item = $datetime->getParent();
     // @todo Remove this in https://www.drupal.org/project/drupal/issues/2958416.
     if ($field_item instanceof DateTimeItem && $field_item->getFieldDefinition()->getFieldStorageDefinition()->getSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
@@ -57,6 +58,10 @@ public function denormalize($data, $class, $format = NULL, array $context = [])
     $field_definition = isset($context['target_instance'])
       ? $context['target_instance']->getFieldDefinition()
       : (isset($context['field_definition']) ? $context['field_definition'] : NULL);
+    if ($field_definition === NULL) {
+      throw new InvalidArgumentException('$context[\'target_instance\'] or $context[\'field_definition\'] must be set to denormalize with the DateTimeIso8601Normalizer');
+    }
+
     $datetime_type = $field_definition->getSetting('datetime_type');
     $is_date_only = $datetime_type === DateTimeItem::DATETIME_TYPE_DATE;