diff --git a/program/lib/Roundcube/rcube_cache.php b/program/lib/Roundcube/rcube_cache.php
index a02493b8c02..895b76fb6f6 100644
--- a/program/lib/Roundcube/rcube_cache.php
+++ b/program/lib/Roundcube/rcube_cache.php
@@ -405,7 +405,7 @@ protected function write_index()
             }
         }
 
-        $data = serialize($this->index);
+        $data = serialize(array_values($this->index));
 
         $this->add_item($this->ikey(), $data);
     }