Index: includes/update.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/update.inc,v
retrieving revision 1.77
diff -u -p -r1.77 update.inc
--- includes/update.inc	12 Oct 2010 02:50:03 -0000	1.77
+++ includes/update.inc	22 Oct 2010 12:09:08 -0000
@@ -125,7 +125,11 @@ function update_prepare_d7_bootstrap() {
     $pdo_link = 'http://drupal.org/requirements/pdo#pecl';
   }
   // Check that the correct driver is loaded for the database being updated.
-  elseif (!in_array($databases['default']['default']['driver'], PDO::getAvailableDrivers())) {
+  // If we have no driver information (for example, if someone tried to create
+  // the Drupal 7 $databases array themselves but did not do it correctly),
+  // this message will be confusing, so do not perform the check; instead, just
+  // let the database connection fail in the code that follows.
+  elseif (isset($databases['default']['default']['driver']) && !in_array($databases['default']['default']['driver'], PDO::getAvailableDrivers())) {
     $message = '<h2>A PDO database driver is required!</h2><p>You need to enable the PDO_' . strtoupper($databases['default']['default']['driver']) . ' database driver for PHP ' . DRUPAL_MINIMUM_PHP . ' or higher so that Drupal 7 can access the database.</p>';
   }
   if ($message) {