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
@@ -35,11 +35,15 @@
 
   /**
    * The complete theme registry array.
+   *
+   * @var array
    */
   protected $completeRegistry;
 
   /**
    * Whether all modules have already been loaded.
+   *
+   * @var bool
    */
   protected $modulesLoaded;
 
@@ -58,7 +62,7 @@
    * @param \Drupal\Core\Lock\LockBackendInterface $lock
    *   The lock backend.
    * @param \Drupal\Core\Theme\ActiveTheme $theme
-   *   The theme object representing the active theme for this registry.
+   *   The theme object representing the active theme for this registry. 
    * @param array $tags
    *   (optional) The tags to specify for the cache item.
    * @param bool $modules_loaded
@@ -110,8 +114,8 @@
       else {
         // If there is no runtime cache stored, fetch the full theme registry,
         // but then initialize each value to NULL. This allows offsetExists()
-        // to function correctly on non-registered theme hooks without triggering
-        // a call to resolveCacheMiss().
+        // to function correctly on non-registered theme hooks without
+        // triggering a call to resolveCacheMiss().
         $this->storage = $this->initializeRegistry();
         foreach (array_keys($this->storage) as $key) {
           $this->persist($key);
@@ -173,7 +177,7 @@
     if (!isset($this->completeRegistry)) {
       $this->completeRegistry = \Drupal::service('theme.registry')->get();
     }
-    $storage = $this->storage;
+    $storage = $this->getStorage();
     $storage[$key] = $this->completeRegistry[$key];
     if ($this->getPersistable()) {
       $this->persist($key);
diff -u b/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
--- b/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -31,7 +31,6 @@
    * Tests the behavior of the theme registry class.
    */
   function testRaceCondition() {
-
     // The theme registry is not marked as persistable in case we don't have a
     // proper request.
     \Drupal::request()->setMethod('GET');
@@ -48,7 +47,7 @@
 
     // Trigger a cache miss for an offset.
     $this->assertTrue($registry->get('theme_test_template_test'), 'Offset was returned correctly from the theme registry.');
-    $this->assertTRUE(\Drupal::cache()->get($cid), 'Cache entry was created.');
+    $this->assertTrue(\Drupal::cache()->get($cid), 'Cache entry was created.');
     // This will cause the ThemeRegistry class to write an updated version of
     // the cache entry when it is destroyed, usually at the end of the request.
     // Before that happens, manually delete the cache entry we created earlier