diff --git a/core/modules/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
index 2c8c862..30dd6c5 100644
--- a/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -571,7 +571,7 @@ protected function registerStreamWrapper($scheme, $class, $type = StreamWrapperI
    *   The rendered string output (typically HTML).
    */
   protected function render(array &$elements) {
-    $content = $this->container->get('renderer')->renderRoot($elements);
+    $content = (string) $this->container->get('renderer')->renderRoot($elements);
     drupal_process_attached($elements);
     $this->setRawContent($content);
     $this->verbose('<pre style="white-space: pre-wrap">' . SafeMarkup::checkPlain($content));
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index 376fed2..9a31376 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -841,7 +841,7 @@ protected function disableModules(array $modules) {
    *   The rendered string output (typically HTML).
    */
   protected function render(array &$elements) {
-    $content = $this->container->get('renderer')->render($elements);
+    $content = (string) $this->container->get('renderer')->render($elements);
     drupal_process_attached($elements);
     $this->setRawContent($content);
     $this->verbose('<pre style="white-space: pre-wrap">' . SafeMarkup::checkPlain($content));