diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index bb345fc..20b2926 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -777,7 +777,7 @@ protected function moduleData($module) {
    * @todo Remove obsolete $module_list parameter. Only $module_filenames is
    *   needed.
    */
-  public function updateModules(array $module_list, array $module_filenames = array(), array $experimental_namespaces = array()) {
+  public function updateModules(array $module_list, array $module_filenames = array(), array $experimental_namespaces = NULL) {
     $pre_existing_module_namespaces = [];
     $pre_existing_experimental_namespaces = [];
     if ($this->booted && is_array($this->moduleList)) {
@@ -789,10 +789,12 @@ public function updateModules(array $module_list, array $module_filenames = arra
       $this->moduleData[$name] = $extension;
     }
 
-    foreach ($experimental_namespaces as $module => $namespaces) {
-      $this->experimentalNamespaces = array_merge($this->experimentalNamespaces, $namespaces);
+    if (!empty($experimental_namespaces)) {
+      foreach ($experimental_namespaces as $module => $namespaces) {
+        $this->experimentalNamespaces = array_merge($this->experimentalNamespaces, $namespaces);
+      }
+      $this->experimentalNamespaces = array_unique($this->experimentalNamespaces);
     }
-    $this->experimentalNamespaces = array_unique($this->experimentalNamespaces);
 
     // If we haven't yet booted, we don't need to do anything: the new module
     // list will take effect when boot() is called. However we set a
diff --git a/core/lib/Drupal/Core/DrupalKernelInterface.php b/core/lib/Drupal/Core/DrupalKernelInterface.php
index 414327b..69edb2d 100644
--- a/core/lib/Drupal/Core/DrupalKernelInterface.php
+++ b/core/lib/Drupal/Core/DrupalKernelInterface.php
@@ -100,9 +100,9 @@ public function getAppRoot();
    * @param array $module_filenames
    *   List of module filenames, keyed by module name.
    * @param array $experimental_namespaces
-   *   List of experimental namespaces, keyed by module name.
+   *   (optional) List of experimental namespaces, keyed by module name.
    */
-  public function updateModules(array $module_list, array $module_filenames = array(), array $experimental_namespaces = array());
+  public function updateModules(array $module_list, array $module_filenames = array(), array $experimental_namespaces = NULL);
 
   /**
    * Force a container rebuild.