diff --git a/GPy/util/caching.py b/GPy/util/caching.py
index b1419aec..196ce343 100644
--- a/GPy/util/caching.py
+++ b/GPy/util/caching.py
@@ -2,6 +2,7 @@
 # Licensed under the BSD 3-clause license (see LICENSE.txt)
 from ..core.parameterization.observable import Observable
 import collections, weakref
+from functools import reduce
 
 class Cacher(object):
     def __init__(self, operation, limit=5, ignore_args=(), force_kwargs=()):