diff --git a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraint.php b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraint.php
index d982193..bf8658a 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraint.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraint.php
@@ -28,4 +28,11 @@ class ValidReferenceConstraint extends Constraint {
    */
   public $message = 'The referenced entity (%type: %id) does not exist.';
 
+  /**
+   * Validation message when the target_id is empty.
+   *
+   * @var string
+   */
+  public $nullMessage = 'This value should not be null.';
+
 }
diff --git a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
index 6a8f7cd..557d0c9 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
@@ -19,6 +19,10 @@ class ValidReferenceConstraintValidator extends ConstraintValidator {
    * {@inheritdoc}
    */
   public function validate($value, Constraint $constraint) {
+    if (!$value->isEmpty && $value->target_id === NULL && !$value->entity->isNew()) {
+      $this->context->addViolation($constraint->nullMessage);
+      return;
+    }
     /* @var \Drupal\Core\Field\FieldItemInterface $value */
     if (!isset($value)) {
       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 3b8cc0e..f456400 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -33,7 +33,7 @@
  *   no_ui = TRUE,
  *   default_formatter = "entity_reference_label",
  *   list_class = "\Drupal\Core\Field\EntityReferenceFieldItemList",
- *   constraints = {"ValidReference" = {}, "EntityReferenceNotNull" = {}}
+ *   constraints = {"ValidReference" = {}}
  * )
  */
 class EntityReferenceItem extends FieldItemBase {
diff --git a/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php b/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
index 4c5d9a4..d97b999 100644
--- a/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
+++ b/core/lib/Drupal/Core/TypedData/DataReferenceTargetDefinition.php
@@ -6,7 +6,18 @@
 
 namespace Drupal\Core\TypedData;
 
-
+/**
+ * A typed data definition class for the entity reference target_id property.
+ *
+ * The target_id property differs from other data definitions in that it is
+ * required at the storage level, but not at the validation level. This is
+ * because its value can be set just-in-time using the preSave() method.
+ *
+ * Validation for the target_id property is provided by the 'ValidReference'
+ * validation constraint.
+ *
+ * @see \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem::preSave()
+ */
 class DataReferenceTargetDefinition extends DataDefinition {
 
   /**
@@ -16,12 +27,12 @@ public function getConstraints() {
     $constraints = parent::getConstraints();
     // If this data definition is marked as required for the sake of schema
     // definitions, we don't enforce it using the NotNull constraint. Instead
-    // \Drupal\Core\Field\EntityReferenceFieldItemList::getConstraints() adds
-    // the 'EntityReferenceNotNull' constraint that operates at the field-item
-    // list level.
+    // \Drupal\Core\Field\EntityReferenceItem is validated by the
+    // 'ValidReference' constraint that operates at the field-item level. This
+    // constraint takes into consideration that the target_id property can
+    // be derived from the entity property.
     unset($constraints['NotNull']);
     return $constraints;
   }
 
-
 }
diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraint.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraint.php
deleted file mode 100644
index 38d3927..0000000
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraint.php
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Validation\Plugin\Validation\Constraint\EntityReferenceNotNullConstraint.
- */
-
-namespace Drupal\Core\Validation\Plugin\Validation\Constraint;
-
-/**
- * EntityReferenceNotNull constraint.
- *
- * @Constraint(
- *   id = "EntityReferenceNotNull",
- *   label = @Translation("EntityReferenceNotNull", context = "Validation"),
- *   type = false
- * )
- */
-class EntityReferenceNotNullConstraint extends NotNullConstraint { }
diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraintValidator.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraintValidator.php
deleted file mode 100644
index e30a161..0000000
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/EntityReferenceNotNullConstraintValidator.php
+++ /dev/null
@@ -1,31 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Validation\Plugin\Validation\Constraint\EntityReferenceNotNullConstraintValidator.
- */
-
-namespace Drupal\Core\Validation\Plugin\Validation\Constraint;
-
-use Drupal\Core\TypedData\Validation\TypedDataAwareValidatorTrait;
-use Symfony\Component\Validator\Constraint;
-use Symfony\Component\Validator\Constraints\NotNullValidator;
-
-/**
- * EntityReferenceNotNull constraint validator.
- */
-class EntityReferenceNotNullConstraintValidator extends NotNullValidator {
-
-  use TypedDataAwareValidatorTrait;
-
-  /**
-   * {@inheritdoc}
-   */
-  public function validate($item, Constraint $constraint) {
-    if (!$item->isEmpty && $item->target_id === NULL && !$item->entity->isNew()) {
-      $this->context->addViolation($constraint->message);
-      return;
-    }
-  }
-
-}