src/EntityToJsonApi.php                                 | 2 +-
 src/ResourceType/ConfigurableResourceTypeRepository.php | 5 +++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/EntityToJsonApi.php b/src/EntityToJsonApi.php
index 57f63aa..6863501 100644
--- a/src/EntityToJsonApi.php
+++ b/src/EntityToJsonApi.php
@@ -103,7 +103,7 @@ class EntityToJsonApi {
       $referenced_entities = array_merge($referenced_entities, $entity->get($field_name)->referencedEntities());
     }
     $document = ConfigurableResourceTypeRepository::isJsonApi2x()
-      ? new $this->classToUse($entity, new NullEntityCollection(), [])
+      ? new $this->classToUse($entity, new EntityCollection($referenced_entities), [])
       : new $this->classToUse($entity);
     return $this->serializer->serialize($document,
       'api_json',
diff --git a/src/ResourceType/ConfigurableResourceTypeRepository.php b/src/ResourceType/ConfigurableResourceTypeRepository.php
index 6732259..97a5684 100644
--- a/src/ResourceType/ConfigurableResourceTypeRepository.php
+++ b/src/ResourceType/ConfigurableResourceTypeRepository.php
@@ -128,8 +128,9 @@ class ConfigurableResourceTypeRepository extends ResourceTypeRepository {
   /**
    * {@inheritdoc}
    *
-   * Mostly the same, with two key differences:
-   * 1. Different class
+   * Mostly the same as the parent implementation, with three key differences:
+   * 1. Different resource type class.
+   * 2. Every resource type is assumed to be mutable.
    * 2. Field mapping not based on logic, but on configuration.
    */
   protected function createResourceType(EntityTypeInterface $entity_type, $bundle) {