diff --git a/src/calibre/library/caches.py b/src/calibre/library/caches.py
index f27e15a74b..21a2622f33 100644
--- a/src/calibre/library/caches.py
+++ b/src/calibre/library/caches.py
@@ -7,7 +7,7 @@ __copyright__ = '2010, Kovid Goyal <kovid@kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
 import re, itertools, time, traceback
-from itertools import repeat
+from itertools import repeat, izip, imap
 from datetime import timedelta
 from threading import Thread
 
@@ -792,9 +792,8 @@ class ResultCache(SearchQueryParser): # {{{
             self.marked_ids_dict = dict.fromkeys(id_dict, u'true')
         else:
             # Ensure that all the items in the dict are text
-            self.marked_ids_dict = {}
-            for id_, val in id_dict.iteritems():
-                self.marked_ids_dict[id_] = unicode(val)
+            self.marked_ids_dict = dict(izip(id_dict.iterkeys(), imap(unicode,
+                id_dict.itervalues())))
 
         # Set the values in the cache
         marked_col = self.FIELD_MAP['marked']