diff -u b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
--- b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
+++ b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
@@ -88,7 +88,7 @@
    *
    * @see \Drupal\system\Tests\Ajax\ElementValidationTest::testAjaxElementValidation()
    */
-  function testAjaxElementValidation() {
+  public function testAjaxElementValidation() {
     $edit = ['drivertext' => t('some dumb text')];
     $this->drupalPostAjaxForm('ajax_validation_test', $edit, 'drivertext');
   }