diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 993b377..169366e 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -1093,12 +1093,11 @@ function _drupal_get_filename_fallback_trigger_error($type, $name, $error_type)
  * cleared in system_list_reset() (i.e. after a module/theme rebuild).
  */
 function drupal_file_scan_write_cache() {
-  // Only write to the persistent cache if we are fully bootstrapped.
-  if (drupal_get_bootstrap_phase() != DRUPAL_BOOTSTRAP_FULL) {
-    return;
-  }
+  // Only write to the persistent cache if requested, and if we know that any
+  // data previously in the cache was successfully loaded and merged in by
+  // _drupal_file_scan_cache().
   $file_scans = &_drupal_file_scan_cache();
-  if (isset($file_scans['#write_cache'])) {
+  if (isset($file_scans['#write_cache']) && isset($file_scans['#cache_merge_done'])) {
     unset($file_scans['#write_cache']);
     cache_set('_drupal_file_scan_cache', $file_scans, 'cache_bootstrap');
   }