diff --git a/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
index 77ce647..f78d4d7 100644
--- a/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -581,6 +581,9 @@ protected function getSchemaWrapper() {
    *
    * @return mixed
    *   The value cast to the type indicated in the schema.
+   *
+   * @throws \Drupal\Core\Config\UnsupportedConfigDataTypeException
+   *   Exception on unsupported/undefined data type deducted.
    */
   protected function castValue($key, $value) {
     if ($value === NULL) {