diff -u b/core/lib/Drupal/Core/Utility/ThemeRegistry.php b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
--- b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
+++ b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
@@ -125,7 +125,7 @@
   /**
    * Initializes the full theme registry.
    *
-   * @return
+   * @return mixed[]
    *   An array with the keys of the full theme registry, but the values
    *   initialized to NULL.
    */
@@ -151,14 +151,18 @@
    * {@inheritdoc}
    */
   public function get($key) {
+    // When storage is not set get the storage.
+    if (!$this->storage) {
+      $this->getStorage();
+    }
+
     // If the offset is set but empty, it is a registered theme hook that has
     // not yet been requested. Offsets that do not exist at all were not
     // registered in hook_theme().
-    $storage = $this->getStorage();
-    if (isset($storage[$key])) {
-      return $storage[$key];
+    if (isset($this->storage[$key])) {
+      return $this->storage[$key];
     }
-    elseif (array_key_exists($key, $storage)) {
+    elseif (array_key_exists($key, $this->storage)) {
       return $this->resolveCacheMiss($key);
     }
   }
diff -u b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
--- b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -66,6 +66,11 @@
    */
   protected $themeManager;
 
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack|\PHPUnit_Framework_MockObject_MockObject
+   */
   protected $requestStack;
 
   /**