diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 58ee220467..dad1a3a98f 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -386,6 +386,7 @@ class SortKey(object):
             ans = cmp(self.sort_key[i], other.sort_key[i])
             if ans != 0:
                 return ans * order
+        return 0
 
 
 # Testing {{{