diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index b596092..2146391 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1311,7 +1311,7 @@ protected function curlHeaderCallback($curlHandler, $header) {
       // the header.
       call_user_func_array(array(&$this, 'error'), unserialize(urldecode($matches[1])));
     }
-    if (preg_match('/^X-Drupal-Memory-Peak: (.*)$/', $header, $matches)) {
+    if (!$this->ignoreMemoryUsage && preg_match('/^X-Drupal-Memory-Peak: (.*)$/', $header, $matches)) {
       if ((float) $matches[1] >= $this->memoryUsageTrigger) {
         $this->fail(format_string('Child site memory usage (@siteMB) exceeded acceptable maximum (@maxMB) on url @url', [
           '@url' => $this->getUrl(),