diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 5e19805..d25efb2 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -439,7 +439,7 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
       'base hook' => TRUE,
     );
 
-    $module_list = array_keys((array) $this->moduleHandler->getModuleList());
+    $module_list = array_keys($this->moduleHandler->getModuleList());
 
     // Invoke the hook_theme() implementation, preprocess what is returned, and
     // merge it into $cache.
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 4a9101d..0cb5a25 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -105,6 +105,9 @@ public function testGetRegistryForModule() {
       ->method('getImplementations')
       ->with('theme')
       ->will($this->returnValue(array('theme_test')));
+    $this->moduleHandler->expects($this->atLeastOnce())
+      ->method('getModuleList')
+      ->willReturn([]);
 
     $registry = $this->registry->get();