diff --git a/core/lib/Drupal/Core/Entity/Plugin/DataType/EntityReferenceItem.php b/core/lib/Drupal/Core/Entity/Plugin/DataType/EntityReferenceItem.php
index d529120..a9a62f3 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/DataType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/DataType/EntityReferenceItem.php
@@ -53,6 +53,8 @@ public function getPropertyDefinitions() {
       $target_type_info = \Drupal::entityManager()->getDefinition($target_type);
       if (is_subclass_of($target_type_info['class'], '\Drupal\Core\Entity\ContentEntityInterface')) {
         static::$propertyDefinitions[$key]['target_id'] = array(
+          // @todo: Lookup the entity type's ID data type and use it here.
+          // https://drupal.org/node/2107249
           'type' => 'integer',
           'label' => t('Entity ID'),
           'constraints' => array(