diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index a8e99d6..32e6e5e 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1787,25 +1787,24 @@ protected function drupalPostForm($path, $edit, $submit, array $options = array(
    * @see ajax.js
    */
   protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_path = NULL, array $options = array(), array $headers = array(), $form_html_id = NULL, $ajax_settings = NULL) {
-    if (isset($options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE])) {
-      $original_wrapper = $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE];
-      unset($options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE]);
-    }
 
     // Get the content of the initial page prior to calling drupalPostForm(),
     // since drupalPostForm() replaces $this->content.
     if (isset($path)) {
-      $this->drupalGet($path, $options);
+      // Avoid sending the wrapper query argument to drupalGet so we can fetch
+      // the form and populate the internal WebTest values.
+      $get_options = $options;
+      unset($get_options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE]);
+      $this->drupalGet($path, $get_options);
     }
     $content = $this->content;
     $drupal_settings = $this->drupalSettings;
 
-    if (isset($original_wrapper)) {
-      $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] = $original_wrapper;
-    }
-    else {
-      $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] = 'drupal_ajax';
-    }
+    // Provide a default value for the wrapper envelope.
+    $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] =
+      isset($options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE]) ?
+        $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] :
+        'drupal_ajax';
 
     // Get the Ajax settings bound to the triggering element.
     if (!isset($ajax_settings)) {
diff --git a/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
index 663a904..fe9ff94 100644
--- a/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
+++ b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
@@ -120,7 +120,6 @@ public function dialog() {
       '#url' => Url::fromRoute('ajax_test.dialog_contents'),
       '#attributes' => array(
         'class' => array('use-ajax'),
-        'data-accepts' => 'application/vnd.drupal-modal', // TODO replace this.
       ),
     );