diff -u b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
--- b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -87,7 +87,7 @@
    * {@inheritdoc}
    */
   public static function open(array &$connection_options = array()) {
-    if (isset($connection_options['#dsn_utf8_fallback']) && $connection_options['#dsn_utf8_fallback'] === TRUE) {
+    if (isset($connection_options['_dsn_utf8_fallback']) && $connection_options['_dsn_utf8_fallback'] === TRUE) {
       // Only used during the installer version check, as a fallback from utf8mb4.
       $charset = 'utf8';
     }
diff -u b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
--- b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
@@ -77,7 +77,7 @@
           $info_copy = $info;
           // Set a flag to fall back to utf8. Note: this flag should only be
           // used here and is for internal use only.
-          $info_copy['default']['#dsn_utf8_fallback'] = TRUE;
+          $info_copy['default']['_dsn_utf8_fallback'] = TRUE;
           // In order to change the Database::$databaseInfo array, we need to
           // remove the active connection, then re-add it with the new info.
           Database::removeConnection('default');