diff --git a/src/Controller/EntityResource.php b/src/Controller/EntityResource.php
index ba0aec6..217d8ea 100644
--- a/src/Controller/EntityResource.php
+++ b/src/Controller/EntityResource.php
@@ -488,7 +488,10 @@ class EntityResource {
   public function getRelationship(ResourceType $resource_type, FieldableEntityInterface $entity, $related, Request $request, $response_code = 200) {
     /* @var \Drupal\Core\Field\EntityReferenceFieldItemListInterface $field_list */
     $field_list = $entity->get($resource_type->getInternalName($related));
-    $response = $this->buildWrappedResponse($field_list, $request, $this->getIncludes($request, $entity), $response_code);
+    // Access will have already been checked by the RelationshipFieldAccess
+    // service, so we don't need to call ::getAccessCheckedResourceObject().
+    $resource_object = new ResourceObject($resource_type, $entity);
+    $response = $this->buildWrappedResponse($field_list, $request, $this->getIncludes($request, $resource_object), $response_code);
     // Add the host entity as a cacheable dependency.
     $response->addCacheableDependency($entity);
     return $response;
diff --git a/src/IncludeResolver.php b/src/IncludeResolver.php
index fcf4973..d290ca4 100644
--- a/src/IncludeResolver.php
+++ b/src/IncludeResolver.php
@@ -59,7 +59,7 @@ class IncludeResolver {
   public function resolve(ResourceType $base_resource_type, $data, $include_parameter, $related_field = NULL) {
     assert($data instanceof ResourceIdentifierInterface || $data instanceof EntityCollection);
     // Map a single entity into an EntityCollection.
-    $entity_collection = $data instanceof ResourceObject ? new EntityCollection([$data], 1) : $data;
+    $entity_collection = $data instanceof ResourceIdentifierInterface ? new EntityCollection([$data], 1) : $data;
     $include_tree = static::toIncludeTree($base_resource_type, $include_parameter, $related_field);
     return EntityCollection::deduplicate($this->resolveIncludeTree($include_tree, $entity_collection));
   }
diff --git a/src/JsonApiResource/EntityCollection.php b/src/JsonApiResource/EntityCollection.php
index 39fa89a..8f67f1e 100644
--- a/src/JsonApiResource/EntityCollection.php
+++ b/src/JsonApiResource/EntityCollection.php
@@ -4,7 +4,6 @@ namespace Drupal\jsonapi\JsonApiResource;
 
 use Drupal\Component\Assertion\Inspector;
 use Drupal\jsonapi\Exception\EntityAccessDeniedHttpException;
-use Drupal\jsonapi\LabelOnlyEntity;
 
 /**
  * Wrapper to normalize collections with multiple entities.
@@ -44,7 +43,7 @@ class EntityCollection implements \IteratorAggregate, \Countable {
   /**
    * Instantiates a EntityCollection object.
    *
-   * @param \Drupal\Core\Entity\EntityInterface|null[]|false[] $resource_objects
+   * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifierInterface[]|null[]|false[] $resource_objects
    *   The resources for the collection.
    * @param int $cardinality
    *   The number of resources that this collection may contain. Related
@@ -56,9 +55,7 @@ class EntityCollection implements \IteratorAggregate, \Countable {
     assert(Inspector::assertAll(function ($entity) {
         return $entity === NULL
         || $entity === FALSE
-        || $entity instanceof ResourceObject
-        || $entity instanceof LabelOnlyEntity
-        || $entity instanceof EntityAccessDeniedHttpException;
+        || $entity instanceof ResourceIdentifierInterface;
     }, $resource_objects));
     assert($cardinality >= -1 && $cardinality !== 0, 'Cardinality must be -1 for unlimited cardinality or a positive integer.');
     assert($cardinality === -1 || count($resource_objects) <= $cardinality, 'If cardinality is not unlimited, the number of given resources must not exceed the cardinality of the collection.');