diff -u b/core/includes/update.inc b/core/includes/update.inc
--- b/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -135,9 +135,12 @@
     drupal_install_config_directories();
   }
 
-  // Bootstrap the database.
+  // Bootstrap the database. During this, the DRUPAL_BOOTSTRAP_PAGE_CACHE will
+  // try to read the cache but the cache tables might not be Drupal 8
+  // compatible yet. Use the null backend by default to avoid exceptions.
   $GLOBALS['conf']['cache_classes'] = array('cache' => 'Drupal\Core\Cache\NullBackend');
   drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
+  // Now remove the cache override.
   unset($GLOBALS['conf']['cache_classes']['cache']);
   drupal_static_reset('cache');