diff --git i/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php w/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
index 0e7ab72..b1fc7e8 100644
--- i/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ w/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -43,7 +43,7 @@ class Connection extends DatabaseConnection {
     }
     else {
       // Default to TCP connection on port 3306.
-      $dsn = 'mysql:host=' . $connection_options['host'] . ';port=' . (empty($connection_options['port']) ? 3306 : $connection_options['port']);
+      $dsn = 'mysql:host=' . (empty($connection_options['host']) ? 'localhost' : $connection_options['host']) . ';port=' . (empty($connection_options['port']) ? 3306 : $connection_options['port']);
     }
     $dsn .= ';dbname=' . $connection_options['database'];
     // Allow PDO options to be overridden.