diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 58d0ac9..6fea484 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -310,11 +310,11 @@ public function render($hook, array $variables) {
     // Generate the output using either a function or a template.
     $output = '';
     if (isset($info['function'])) {
-      if (is_callable($info['function'])) {
+      if (function_exists($info['function'])) {
         // Theme functions do not render via the theme engine, so the output is
         // not autoescaped. However, we can only presume that the theme function
         // has been written correctly and that the markup is safe.
-        $output = Markup::create(call_user_func_array($info['function'], array($variables)));
+        $output = Markup::create($info['function']($variables));
       }
     }
     else {