diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 3165815b17..8a2b1d6b7f 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -120,6 +120,9 @@ protected function prepareSettings() {
       // Add a listener to validate configuration schema on save.
       $yaml = new SymfonyYaml();
       $content = file_get_contents($directory . '/services.yml');
+      // @todo Remove preg_replace() once
+      //   https://github.com/symfony/symfony/pull/25787 is in Symfony 3.4.
+      $content = preg_replace('/:$\n^\s+{\s*}$/m', ': {}', $content);
       $services = $yaml->parse($content);
       $services['services']['simpletest.config_schema_checker'] = [
         'class' => ConfigSchemaChecker::class,