.../src/Normalizer/DateTimeIso8601Normalizer.php   |  6 ++++-
 .../src/Normalizer/DateTimeNormalizer.php          |  6 ++++-
 .../Normalizer/DateTimeIso8601NormalizerTest.php   | 30 ++++++++++++++++++++++
 .../src/Unit/Normalizer/DateTimeNormalizerTest.php | 11 ++++++++
 4 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index 8138639..4515e52 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -42,7 +42,11 @@ class DateTimeIso8601Normalizer extends DateTimeNormalizer {
   public function normalize($datetime, $format = NULL, array $context = []) {
     $field_item = $datetime->getParent();
     if ($field_item instanceof DateTimeItem && $field_item->getFieldDefinition()->getFieldStorageDefinition()->getSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
-      return $datetime->getDateTime()->format($this->allowedFormats['date-only']);
+      $drupal_date_time = $datetime->getDateTime();
+      if ($drupal_date_time === NULL) {
+        return $drupal_date_time;
+      }
+      return $drupal_date_time->format($this->allowedFormats['date-only']);
     }
     return parent::normalize($datetime, $format, $context);
   }
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index 59ca022..89e4e86 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -46,7 +46,11 @@ class DateTimeNormalizer extends NormalizerBase implements DenormalizerInterface
    * {@inheritdoc}
    */
   public function normalize($datetime, $format = NULL, array $context = []) {
-    return $datetime->getDateTime()
+    $drupal_date_time = $datetime->getDateTime();
+    if ($drupal_date_time === NULL) {
+      return $drupal_date_time;
+    }
+    return $drupal_date_time
       // Set an explicit timezone. Otherwise, timestamps may end up being
       // normalized using the user's preferred timezone. Which would result in
       // many variations and complex caching.
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
index c630deb..b3cf2ae 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
@@ -123,6 +123,36 @@ public function testNormalize($parent_field_item_class, $datetime_type, $expecte
   }
 
   /**
+   * @covers ::normalize
+   * @dataProvider providerTestNormalize
+   */
+  public function testNormalizeWhenNull($parent_field_item_class, $datetime_type, $expected_format) {
+    $field_item = $this->prophesize($parent_field_item_class);
+    if ($parent_field_item_class === DateTimeItem::class) {
+      $field_storage_definition = $this->prophesize(FieldStorageDefinitionInterface::class);
+      $field_storage_definition->getSetting('datetime_type')
+        ->willReturn($datetime_type);
+      $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+      $field_definition->getFieldStorageDefinition()
+        ->willReturn($field_storage_definition);
+      $field_item->getFieldDefinition()
+        ->willReturn($field_definition);
+    }
+    else {
+      $field_item->getFieldDefinition(Argument::any())
+        ->shouldNotBeCalled();
+    }
+    $this->data->getParent()
+      ->willReturn($field_item);
+
+    $this->data->getDateTime()
+      ->willReturn(NULL);
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertNull($normalized);
+  }
+
+  /**
    * Data provider for testNormalize.
    *
    * @return array
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
index f37f75d..570784b 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
@@ -95,6 +95,17 @@ public function testNormalize() {
   }
 
   /**
+   * @covers ::normalize
+   */
+  public function testNormalizeWhenNull() {
+    $this->data->getDateTime()
+      ->willReturn(NULL);
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertNull($normalized);
+  }
+
+  /**
    * Tests the denormalize function with good data.
    *
    * @covers ::denormalize