diff --git a/core/lib/Drupal/Core/Render/Element/Radio.php b/core/lib/Drupal/Core/Render/Element/Radio.php
index 4eb34ac..5a115fb 100644
--- a/core/lib/Drupal/Core/Render/Element/Radio.php
+++ b/core/lib/Drupal/Core/Render/Element/Radio.php
@@ -49,7 +49,7 @@ public function getInfo() {
   protected static function castValue($value)
   {
     if (is_bool($value) || (string) (int) $value === (string) $value) {
-        return (string) (int) $index;
+        return (string) (int) $value;
     }
   }