diff --git a/core/lib/Drupal/Core/Config/StorableConfigBase.php b/core/lib/Drupal/Core/Config/StorableConfigBase.php
index 24bc4d2..5c6e8b3 100644
--- a/core/lib/Drupal/Core/Config/StorableConfigBase.php
+++ b/core/lib/Drupal/Core/Config/StorableConfigBase.php
@@ -136,8 +136,15 @@ protected function getSchemaWrapper() {
   /**
    * Validate the values are allowed data types.
    *
-   * @throws UnsupportedDataTypeConfigException
-   *   If there is any invalid value.
+   * @param string $key
+   *   A string that maps to a key within the configuration data.
+   * @param string $value
+   *   Value to associate with the key.
+   *
+   * @return null
+   *
+   * @throws \Drupal\Core\Config\UnsupportedDataTypeConfigException
+   *   If the value is unsupported in configuration.
    */
   protected function validateValue($key, $value) {
     // Minimal validation. Should not try to serialize resources or non-arrays.
@@ -166,12 +173,15 @@ protected function validateValue($key, $value) {
    *   The value cast to the type indicated in the schema.
    *
    * @throws \Drupal\Core\Config\UnsupportedDataTypeConfigException
-   *   Exception on unsupported/undefined data type deducted.
+   *   If the value is unsupported in configuration.
    */
   protected function castValue($key, $value) {
     $element = $this->getSchemaWrapper()->get($key);
     // Do not cast value if it is unknown or defined to be ignored.
     if ($element && ($element instanceof Undefined || $element instanceof Ignore)) {
+      // Do validate the value (may throw UnsupportedDataTypeConfigException)
+      // to ensure unsupported types are not supported in this case either.
+      $this->validateValue($key, $value);
       return $value;
     }
     if (is_scalar($value) || $value === NULL) {