diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index a2c43cd..4e7ad80 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -463,6 +463,7 @@ function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
   // merge it into $cache.
   $function = $name . '_theme';
   if (function_exists($function)) {
+    // Cast to an array to not fail an empty hook_theme implementations.
     $result = (array) $function($cache, $type, $theme, $path);
     foreach ($result as $hook => $info) {
       // When a theme or engine overrides a module's theme function