diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 6578fae..4ddce53 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -907,11 +907,33 @@ protected function setUp() {
     chmod($directory, 0777);
 
     $request = \Drupal::request();
+    // If there is a session manager, close and save the session.
+    $session_manager_state = 0;
+    if (\Drupal::getContainer()->initialized('session_manager')) {
+      $session_manager = \Drupal::service('session_manager');
+      if ($session_manager->isStartedLazy()) {
+        $session_manager_state |= 0x1;
+      }
+      if ($session_manager->isStarted()) {
+        $session_manager_state |= 0x2;
+      }
+      $session_manager->save();
+      unset($session_manager);
+    }
+
     $this->kernel = DrupalKernel::createFromRequest($request, drupal_classloader(), 'prod', TRUE);
+    $this->kernel->prepareLegacyRequest($request);
     // Force the container to be built from scratch instead of loaded from the
     // disk. This forces us to not accidently load the parent site.
     $container = $this->kernel->rebuildContainer();
-    $this->kernel->prepareLegacyRequest($request);
+
+    if ($session_manager_state & 0x1) {
+      $container->get('session_manager')->startLazy();
+    }
+    if ($session_manager_state & 0x2) {
+      $container->get('session_manager')->start();
+    }
+
     $config = $container->get('config.factory');
 
     // Manually create and configure private and temporary files directories.