reverted:
--- b/core/lib/Drupal/Core/Entity/EntityManager.php
+++ a/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -9,7 +9,6 @@
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Component\Plugin\Exception\PluginNotFoundException;
-use Drupal\Component\Uuid\Uuid;
 use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
@@ -969,10 +968,6 @@
    * {@inheritdoc}
    */
   public function loadEntityByUuid($entity_type_id, $uuid) {
-    // Return NULL for invalid UUID's.
-    if (empty($uuid) || !Uuid::isValid($uuid)) {
-      return NULL;
-    }
     $entity_type = $this->getDefinition($entity_type_id);
 
     if (!$uuid_key = $entity_type->getKey('uuid')) {
@@ -981,10 +976,6 @@
 
     $entities = $this->getStorage($entity_type_id)->loadByProperties(array($uuid_key => $uuid));
 
-    if (!reset($entities)) {
-      return NULL;
-    }
-
     return reset($entities);
   }
 
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php
@@ -1545,6 +1545,37 @@ protected function getTestHandlerClass() {
     return get_class($this->getMockForAbstractClass('Drupal\Core\Entity\EntityHandlerBase'));
   }
 
+  /**
+   * Tests the loadEntityByUuid() method.
+   *
+   * @covers ::loadEntityByUuid()
+   * 
+   * @dataProvider providerTestLoadEntityByUuid
+   */
+  public function testloadEntityByUuid($entity_type_id, $uuid) {
+    $entity = $this->getMock('Drupal\Core\Entity\EntityManager');
+    $this->setUpEntityManager(array(
+      'apple' => $entity,
+      'banana' => $entity,
+    ));
+
+    $new_entity = $this->entityManager->loadEntityByUuid($entity_type_id,$uuid);
+    $this->assertNull($new_entity);
+  }
+  
+  /**
+   * Provides test data for testloadEntityByUuid().
+   *
+   * @return array
+   *   Test data.
+   */
+  public function providerTestLoadEntityByUuid() {
+    return array(
+      array('apple', 2415),
+      array('banana', NULL),
+    );
+  }
+
 }
 
 /*