diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1094,8 +1094,13 @@
     'value'    => drupal_hash_base64(drupal_random_bytes(55)),
     'required' => TRUE,
   );
+  $extras = '';
+  if (!empty($form_state['storage']['database']['custom'])) {
+    $driver = $form_state['storage']['database']['driver'];
+    $extras = "drupal_classloader_register('$driver', \"db_drivers/$driver\");\n";
+  }
 
-  drupal_rewrite_settings($settings);
+  drupal_rewrite_settings($settings, $extras);
 
   // Add the config directories to settings.php.
   drupal_install_config_directories();
diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -172,8 +172,10 @@
  *
  * @param $settings
  *   An array of settings that need to be updated.
+ * @param $extras
+ *   Additional code to be added to settings.php.
  */
-function drupal_rewrite_settings($settings = array()) {
+function drupal_rewrite_settings($settings = array(), $extras = '') {
   drupal_static_reset('conf_path');
   $settings_file = conf_path(FALSE) . '/settings.php';
 
@@ -223,6 +225,9 @@
         $buffer .= "\$$setting = " . var_export($data['value'], TRUE) . ";\n";
       }
     }
+    if ($extras) {
+      $buffer .= "\n" . $extras;
+    }
 
     // Write the new settings file.
     if (file_put_contents(DRUPAL_ROOT . '/' . $settings_file, $buffer) === FALSE) {
diff -u b/core/lib/Drupal/Core/Database/Database.php b/core/lib/Drupal/Core/Database/Database.php
--- b/core/lib/Drupal/Core/Database/Database.php
+++ b/core/lib/Drupal/Core/Database/Database.php
@@ -375,7 +375,6 @@
     }
 
     if (!empty(self::$databaseInfo[$key][$target]['custom'])) {
-      drupal_classloader_register($driver, "db_drivers/$driver");
       $driver_class = "Drupal\\{$driver}\\Connection";
     }
     else {