diff --git a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
index 86ecf4d..8f704c1 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
@@ -24,6 +24,9 @@ public function validate($value, Constraint $constraint) {
     if (!isset($value)) {
       return;
     }
+    // We don't use a regular NotNull constraint for the target_id property as
+    // a NULL value is valid if the entity property contains an unsaved entity.
+    // @see \Drupal\Core\TypedData\DataReferenceTargetDefinition::getConstraints
     if (!$value->isEmpty() && $value->target_id === NULL && !$value->entity->isNew()) {
       $this->context->addViolation($constraint->nullMessage);
       return;
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
index d1b2a56..18c0506 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -227,7 +227,10 @@ public function onChange($property_name, $notify = TRUE) {
    */
   public function isEmpty() {
     // Avoid loading the entity by first checking the 'target_id'.
-    if ($this->target_id !== NULL || (($entity = $this->entity) && $entity instanceof EntityInterface)) {
+    if ($this->target_id !== NULL) {
+      return FALSE;
+    }
+    if ($this->entity && $this->entity instanceof EntityInterface) {
       return FALSE;
     }
     return TRUE;
diff --git a/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php b/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
index d97b999..b360986 100644
--- a/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
+++ b/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
@@ -1,7 +1,7 @@
 <?php
 /**
  * @file
- * Contains DataReferenceTargetDefinition.php
+ * Contains \Drupal\Core\TypedData\DataReferenceTargetDefinition.
  */
 
 namespace Drupal\Core\TypedData;