src/Normalizer/EntityNormalizer.php                 | 21 ++++++++++++---------
 .../Unit/Normalizer/ConfigEntityNormalizerTest.php  |  4 +++-
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/src/Normalizer/EntityNormalizer.php b/src/Normalizer/EntityNormalizer.php
index 8727a48..c3d8877 100644
--- a/src/Normalizer/EntityNormalizer.php
+++ b/src/Normalizer/EntityNormalizer.php
@@ -4,7 +4,6 @@ namespace Drupal\jsonapi\Normalizer;
 
 use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
-use Drupal\Core\Field\EntityReferenceFieldItemListInterface;
 use Drupal\Core\TypedData\TypedDataInternalPropertiesHelper;
 use Drupal\jsonapi\Normalizer\Value\EntityNormalizerValue;
 use Drupal\jsonapi\Normalizer\Value\FieldNormalizerValueInterface;
@@ -97,21 +96,25 @@ class EntityNormalizer extends NormalizerBase implements DenormalizerInterface {
     $normalizer_values = [];
     $relationship_field_names = array_keys($resource_type->getRelatableResourceTypes());
     foreach ($this->getFields($entity, $bundle, $resource_type) as $field_name => $field) {
+      $normalized_field = $this->serializeField($field, $context, $format);
+      assert($normalized_field instanceof FieldNormalizerValueInterface);
+
       $in_sparse_fieldset = in_array($field_name, $field_names);
       $is_relationship_field = in_array($field_name, $relationship_field_names);
       // Omit fields not listed in sparse fieldsets, except if they're fields
       // modeling relationships; despite a relationship field being omitted,
       // using `?include` to include related resources is still allowed.
-      if (!$in_sparse_fieldset && !$is_relationship_field) {
+      if (!$in_sparse_fieldset) {
+        if ($is_relationship_field) {
+          $is_null_field = $field instanceof NullFieldNormalizerValue;
+          $has_includes = !empty($normalized_field->getIncludes());
+          if (!$is_null_field && $has_includes) {
+            $normalizer_values[$field_name] = new IncludeOnlyRelationshipNormalizerValue($normalized_field);
+          }
+        }
         continue;
       }
-      $normalized_field = $this->serializeField($field, $context, $format);
-      assert($normalized_field instanceof FieldNormalizerValueInterface);
-      $is_null_field = $field instanceof NullFieldNormalizerValue;
-      $has_includes = !empty($normalized_field->getIncludes());
-      $normalizer_values[$field_name] = !$in_sparse_fieldset && !$is_null_field && $has_includes
-        ? new IncludeOnlyRelationshipNormalizerValue($normalized_field)
-        : $normalized_field;
+      $normalizer_values[$field_name] = $normalized_field;
     }
 
     $link_context = ['link_manager' => $this->linkManager];
diff --git a/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php b/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
index 3d1aab1..27fff3e 100644
--- a/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
+++ b/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
@@ -32,9 +32,11 @@ class ConfigEntityNormalizerTest extends UnitTestCase {
   public function setUp() {
     $link_manager = $this->prophesize(LinkManager::class);
 
+    $resource_type = new ResourceType('dolor', 'sid', NULL);
+    $resource_type->setRelatableResourceTypes([]);
     $resource_type_repository = $this->prophesize(ResourceTypeRepository::class);
     $resource_type_repository->get(Argument::type('string'), Argument::type('string'))
-      ->willReturn(new ResourceType('dolor', 'sid', NULL));
+      ->willReturn($resource_type);
 
     $this->normalizer = new ConfigEntityNormalizer(
       $link_manager->reveal(),