diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 8a2b1d6b7f..03990d8e46 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -168,7 +168,11 @@ protected function setContainerParameter($name, $value) {
     $filename = $this->siteDirectory . '/services.yml';
     chmod($filename, 0666);
 
-    $services = Yaml::decode(file_get_contents($filename));
+    // @todo Remove preg_replace() once
+    //   https://github.com/symfony/symfony/pull/25787 is in Symfony 3.4.
+    $content = file_get_contents($filename);
+    $content = preg_replace('/:$\n^\s+{\s*}$/m', ': {}', $content);
+    $services = Yaml::decode($content);
     $services['parameters'][$name] = $value;
     file_put_contents($filename, Yaml::encode($services));