diff --git a/core/lib/Drupal/Core/Render/Element/Checkbox.php b/core/lib/Drupal/Core/Render/Element/Checkbox.php
index 6a99eca..ea5e9f8 100644
--- a/core/lib/Drupal/Core/Render/Element/Checkbox.php
+++ b/core/lib/Drupal/Core/Render/Element/Checkbox.php
@@ -51,7 +51,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       // NULL to 0, because FormBuilder::handleInputElement() would otherwise
       // replace NULL with empty string, but an empty string is a potentially
       // valid value for a checked checkbox.
-      return isset($element['#default_value']) ? $element['#default_value'] : 0;
+      return TRUE;
     }
     else {
       // Checked checkboxes are submitted with a value (possibly '0' or ''):