diff --git a/GPy/core/parameterization/parameter_core.py b/GPy/core/parameterization/parameter_core.py
index 02cb0a12..b38d9678 100644
--- a/GPy/core/parameterization/parameter_core.py
+++ b/GPy/core/parameterization/parameter_core.py
@@ -18,6 +18,7 @@ import numpy as np
 import re
 import logging
 from .updateable import Updateable
+from functools import reduce
 
 class HierarchyError(Exception):
     """