diff --git a/src/Symfony/Bundle/TwigBundle/CHANGELOG.md b/src/Symfony/Bundle/TwigBundle/CHANGELOG.md
index 1be455e4e8..d25f9aae79 100644
--- a/src/Symfony/Bundle/TwigBundle/CHANGELOG.md
+++ b/src/Symfony/Bundle/TwigBundle/CHANGELOG.md
@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+4.4.0
+-----
+
+ * marked the `TemplateIterator` as `internal`
+
 4.2.0
 -----
 
diff --git a/src/Symfony/Bundle/TwigBundle/CacheWarmer/TemplateCacheWarmer.php b/src/Symfony/Bundle/TwigBundle/CacheWarmer/TemplateCacheWarmer.php
index cb9af8a462..6150e5c1ca 100644
--- a/src/Symfony/Bundle/TwigBundle/CacheWarmer/TemplateCacheWarmer.php
+++ b/src/Symfony/Bundle/TwigBundle/CacheWarmer/TemplateCacheWarmer.php
@@ -28,7 +28,7 @@ class TemplateCacheWarmer implements CacheWarmerInterface, ServiceSubscriberInte
     private $twig;
     private $iterator;
 
-    public function __construct(ContainerInterface $container, \Traversable $iterator)
+    public function __construct(ContainerInterface $container, iterable $iterator)
     {
         // As this cache warmer is optional, dependencies should be lazy-loaded, that's why a container should be injected.
         $this->container = $container;
diff --git a/src/Symfony/Bundle/TwigBundle/TemplateIterator.php b/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
index 17aca046f8..3ad83ef818 100644
--- a/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
+++ b/src/Symfony/Bundle/TwigBundle/TemplateIterator.php
@@ -18,6 +18,8 @@ use Symfony\Component\HttpKernel\KernelInterface;
  * Iterator for all templates in bundles and in the application Resources directory.
  *
  * @author Fabien Potencier <fabien@symfony.com>
+ *
+ * @internal since Symfony 4.4
  */
 class TemplateIterator implements \IteratorAggregate
 {
@@ -31,7 +33,7 @@ class TemplateIterator implements \IteratorAggregate
      * @param KernelInterface $kernel      A KernelInterface instance
      * @param string          $rootDir     The directory where global templates can be stored
      * @param array           $paths       Additional Twig paths to warm
-     * @param string          $defaultPath The directory where global templates can be stored
+     * @param string|null     $defaultPath The directory where global templates can be stored
      */
     public function __construct(KernelInterface $kernel, string $rootDir, array $paths = [], string $defaultPath = null)
     {
@@ -50,40 +52,46 @@ class TemplateIterator implements \IteratorAggregate
             return $this->templates;
         }
 
-        $this->templates = array_merge(
-            $this->findTemplatesInDirectory($this->rootDir.'/Resources/views'),
-            $this->findTemplatesInDirectory($this->defaultPath, null, ['bundles'])
-        );
+        $templates = $this->findTemplatesInDirectory($this->rootDir.'/Resources/views');
+
+        if (null !== $this->defaultPath) {
+            $templates = array_merge(
+                $templates,
+                $this->findTemplatesInDirectory($this->defaultPath, null, ['bundles'])
+            );
+        }
         foreach ($this->kernel->getBundles() as $bundle) {
             $name = $bundle->getName();
             if ('Bundle' === substr($name, -6)) {
                 $name = substr($name, 0, -6);
             }
 
-            $this->templates = array_merge(
-                $this->templates,
+            $templates = array_merge(
+                $templates,
                 $this->findTemplatesInDirectory($bundle->getPath().'/Resources/views', $name),
-                $this->findTemplatesInDirectory($this->rootDir.'/Resources/'.$bundle->getName().'/views', $name),
-                $this->findTemplatesInDirectory($this->defaultPath.'/bundles/'.$bundle->getName(), $name)
+                $this->findTemplatesInDirectory($this->rootDir.'/Resources/'.$bundle->getName().'/views', $name)
             );
+            if (null !== $this->defaultPath) {
+                $templates = array_merge(
+                    $templates,
+                    $this->findTemplatesInDirectory($this->defaultPath.'/bundles/'.$bundle->getName(), $name)
+                );
+            }
         }
 
         foreach ($this->paths as $dir => $namespace) {
-            $this->templates = array_merge($this->templates, $this->findTemplatesInDirectory($dir, $namespace));
+            $templates = array_merge($templates, $this->findTemplatesInDirectory($dir, $namespace));
         }
 
-        return $this->templates = new \ArrayIterator(array_unique($this->templates));
+        return $this->templates = new \ArrayIterator(array_unique($templates));
     }
 
     /**
      * Find templates in the given directory.
      *
-     * @param string      $dir       The directory where to look for templates
-     * @param string|null $namespace The template namespace
-     *
-     * @return array
+     * @return string[]
      */
-    private function findTemplatesInDirectory($dir, $namespace = null, array $excludeDirs = [])
+    private function findTemplatesInDirectory(string $dir, string $namespace = null, array $excludeDirs = []): array
     {
         if (!is_dir($dir)) {
             return [];