diff --git a/PcfAnnuaire.py b/PcfAnnuaire.py
index 701a7f3..2d8f354 100644
--- a/PcfAnnuaire.py
+++ b/PcfAnnuaire.py
@@ -55,7 +55,7 @@ def cache_key(method, self):
     # append time, so old entries do not get returned forever; (they will still
     # get removed as stale entries are automatically removed after 3 days)
     # (cf plone/memoize/volatile.py)
-    return hash(self) + repr(int(time.time()) / 300)
+    return '%s_%s' % (hash(self), (int(time.time()) / 300))
 
 class PcfAnnuaire(BaseContent):