diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 08fc1bc..5e19805 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -592,7 +592,8 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
    * @param string $hook
    *   The name of the suggestion hook to complete.
    * @param array $cache
-   *   The theme registry.
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
    */
   protected function completeSuggestion($hook, array &$cache) {
     $previous_hook = $hook;
@@ -626,7 +627,8 @@ protected function completeSuggestion($hook, array &$cache) {
    * Completes the theme registry adding discovered functions and hooks.
    *
    * @param array $cache
-   *   The theme registry.
+   *   The theme registry as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
    * @param \Drupal\Core\Theme\ActiveTheme $theme
    *   Current active theme.
    *
@@ -637,7 +639,7 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
 
     // Gather prefixes. This will be used to limit the found functions to the
     // expected naming conventions.
-    $prefixes = array_keys($this->moduleHandler->getModuleList());
+    $prefixes = array_keys((array) $this->moduleHandler->getModuleList());
     foreach (array_reverse($theme->getBaseThemes()) as $base) {
       $prefixes[] = $base->getName();
     }