diff --git a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
index 23f0d03..268a8fa 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
@@ -27,6 +27,9 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, array &$form_state) {
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
     $conf_path = './' . conf_path(FALSE);
     $settings_file = $conf_path . '/settings.php';
 
@@ -115,6 +118,9 @@ public function buildForm(array $form, array &$form_state) {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, array &$form_state) {
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
     $driver = $form_state['values']['driver'];
     $database = $form_state['values'][$driver];
     $drivers = drupal_get_database_types();
@@ -137,6 +143,9 @@ public function validateForm(array &$form, array &$form_state) {
   public function submitForm(array &$form, array &$form_state) {
     global $install_state;
 
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
     // Update global settings array and save.
     $settings = array();
     $database = $form_state['storage']['database'];