diff -u b/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
--- b/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1526,7 +1526,7 @@
    * @return \Drupal\Core\Config\Config
    *   The configuration object with original configuration data.
    */
-  public function config($name) {
+  protected function config($name) {
     $config_factory = \Drupal::configFactory();
     $old_state = $config_factory->getOverrideState();
     $config_factory->setOverrideState(FALSE);