diff -u b/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
--- b/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -151,7 +151,7 @@
     $messages = array();
     foreach ($this->results as $result => $success) {
       if (!$success) {
-        $messages[] .= SafeMarkup::escape($result);
+        $messages[] = SafeMarkup::escape($result);
       }
     }
     if (!empty($messages)) {