diff --git a/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
index ee9c23e..94ce095 100644
--- a/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -211,6 +211,7 @@ protected function checkEngineVersion() {
    *   The options form array.
    */
   public function getFormOptions(array $database) {
+    $form['#tree'] = FALSE;
     $form['database'] = array(
       '#type' => 'textfield',
       '#title' => t('Database name'),
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index a24330d..6ab892a 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -689,7 +689,7 @@ public function prepareForm($form_id, &$form, FormStateInterface &$form_state) {
     }
 
     $form += $this->elementInfo->getInfo('form');
-    $form += array('#tree' => FALSE, '#parents' => array());
+    $form += array('#tree' => TRUE, '#parents' => array());
     $form['#validate'][] = '::validateForm';
     $form['#submit'][] = '::submitForm';
 
diff --git a/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php b/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
index 91ed3f0..66fe767 100644
--- a/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
@@ -30,6 +30,7 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state, $install_state = NULL) {
+    $form['#tree'] = FALSE;
     if (count($install_state['translations']) > 1) {
       $files = $install_state['translations'];
     }
diff --git a/core/lib/Drupal/Core/Installer/Form/SelectProfileForm.php b/core/lib/Drupal/Core/Installer/Form/SelectProfileForm.php
index 9be9d13..0d4a418 100644
--- a/core/lib/Drupal/Core/Installer/Form/SelectProfileForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SelectProfileForm.php
@@ -26,6 +26,7 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state, $install_state = NULL) {
+    $form['#tree'] = FALSE;
     $form['#title'] = $this->t('Select an installation profile');
 
     $profiles = array();
diff --git a/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php b/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
index 5634000..0d22fa0 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
@@ -123,6 +123,7 @@ protected function getEditableConfigNames() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['#tree'] = FALSE;
     $form['#title'] = $this->t('Configure site');
 
     // Warn about settings.php permissions risk
diff --git a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
index 4d0f9fe..b9ba76c 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
@@ -55,6 +55,7 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['#tree'] = FALSE;
     $settings_file = './' . $this->sitePath . '/settings.php';
 
     $form['#title'] = $this->t('Database configuration');