diff --git a/core/modules/system/tests/upgrade/upgrade.test b/core/modules/system/tests/upgrade/upgrade.test
index 52d7800..c644684 100644
--- a/core/modules/system/tests/upgrade/upgrade.test
+++ b/core/modules/system/tests/upgrade/upgrade.test
@@ -97,6 +97,15 @@ abstract class UpgradePathTestCase extends DrupalWebTestCase {
     file_prepare_directory($temp_files_directory, FILE_CREATE_DIRECTORY);
     $this->generatedTestFiles = FALSE;
 
+    // Create and set a new configuration directory and signature key.
+    // The child site automatically adjusts the global $config_directory_name to
+    // a test-prefix-specific directory within the public files directory.
+    // @see config_get_config_directory()
+    $GLOBALS['config_directory_name'] = 'simpletest/' . substr($this->databasePrefix, 10) . '/config';
+    $this->configFileDirectory = $this->originalFileDirectory . '/' . $GLOBALS['config_directory_name'];
+    file_prepare_directory($this->configFileDirectory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
+    $GLOBALS['config_signature_key'] = drupal_hash_base64(drupal_random_bytes(55));
+
     // Log fatal errors.
     ini_set('log_errors', 1);
     ini_set('error_log', $public_files_directory . '/error.log');