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
@@ -334,6 +334,9 @@
     if (empty(self::$databaseInfo[$key][$target])) {
       self::$databaseInfo[$key][$target] = self::parseConnectionInfo($info);
 
+      // The database driver needs to be loadable before module namespaces have
+      // been added to the autoloader, because the list of enabled modules and
+      // the service container definition are typically both in the database.
       if (isset($info['autoload']) && $class_loader) {
         $class_loader->addPsr4($info['namespace'] . '\\', $info['autoload']);
       }