diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 43986a253d..1344ad0fc0 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -692,8 +692,7 @@ protected function curlHeaderCallback($curlHandler, $header) {
     // by \Drupal\simpletest\WebTestBase::error().
     if (preg_match('/^X-Drupal-Assertion-[0-9]+: (.*)$/', $header, $matches)) {
       $parameters = unserialize(urldecode($matches[1]));
-      // Swallow deprecation notices. WebTestBase will not be updated to handle
-      // them.
+      // Handle deprecation notices triggered by system under test.
       if ($parameters[1] === 'User deprecated function') {
         $message = (string) $parameters[0];
         if (!in_array($message, DeprecationListener::getSkippedDeprecations())) {
diff --git a/core/tests/bootstrap.php b/core/tests/bootstrap.php
index 165a8cc401..5114890e33 100644
--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -169,6 +169,9 @@ function drupal_phpunit_populate_class_loader() {
 Handle::register();
 
 // Enable the collection of deprecation notices in isolated PHPUnit tests.
-if ($ser = getenv('SYMFONY_DEPRECATIONS_SERIALIZE')) {
-  DeprecationErrorHandler::collectDeprecations($ser);
+// We need to add this here because the assumptions made in
+// vendor/symfony/phpunit-bridge/bootstrap.php are not true for Drupal. We set
+// PHPUNIT_COMPOSER_INSTALL during isolated test runs.
+if ($file = getenv('SYMFONY_DEPRECATIONS_SERIALIZE')) {
+  DeprecationErrorHandler::collectDeprecations($file);
 }