diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index c656b74..af43793 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -9,7 +9,6 @@
 
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\NestedArray;
-use Drupal\Component\Utility\Settings;
 use Drupal\Component\Utility\String;
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Database\Database;
@@ -1944,6 +1943,8 @@ protected function drupalHead($path, array $options = array(), array $headers =
     $options['absolute'] = TRUE;
     $url = $this->container->get('url_generator')->generateFromPath($path, $options);
     $out = $this->curlExec(array(CURLOPT_NOBODY => TRUE, CURLOPT_URL => $url, CURLOPT_HTTPHEADER => $headers));
+    // Ensure that any changes to variables in the other thread are picked up.
+    $this->refreshVariables();
 
     if ($this->dumpHeaders) {
       $this->verbose('GET request to: ' . $path .