core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php | 2 --
 core/modules/serialization/src/Normalizer/DateTimeNormalizer.php        | 1 +
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index 44e2751d67..6b9e4c5f4b 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -68,7 +68,6 @@ public function denormalize($data, $class, $format = NULL, array $context = [])
     if ($is_date_only) {
       $context['datetime_allowed_formats'] = array_intersect_key($this->allowedFormats, ['date-only' => TRUE]);
       $datetime = parent::denormalize($data, $class, $format, $context);
-      unset($context['datetime_allowed_formats']);
       if (!$datetime instanceof \DateTime) {
         return $datetime;
       }
@@ -87,7 +86,6 @@ public function denormalize($data, $class, $format = NULL, array $context = [])
       $context['datetime_allowed_formats']['backward compatibility — deprecated'] = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
       $datetime = parent::denormalize($data, $class, $format, $context);
     }
-    unset($context['datetime_allowed_formats']);
     if (!$datetime instanceof \DateTime) {
       return $datetime;
     }
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index 8e62c30972..209d15201b 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -55,6 +55,7 @@ public function __construct(ConfigFactoryInterface $config_factory) {
    * {@inheritdoc}
    */
   public function normalize($datetime, $format = NULL, array $context = []) {
+    assert($datetime instanceof DateTimeInterface);
     $drupal_date_time = $datetime->getDateTime();
     if ($drupal_date_time === NULL) {
       return $drupal_date_time;