diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1487,6 +1487,9 @@
  *   An array of information about the current installation state.
  */
 function install_bootstrap_full(&$install_state) {
+  // The early stages of the installer override the cache backend since Drupal
+  // isn't fully set up yet. Here the override is removed so that the standard
+  // cache backend will be used again.
   unset($GLOBALS['conf']['cache_classes']['cache']);
   drupal_static_reset('cache');
   // Clear the module list that was overriden earlier in the process.