diff --git a/skutil/utils/fixes.py b/skutil/utils/fixes.py
index 0b4f4e1..cd6accf 100644
--- a/skutil/utils/fixes.py
+++ b/skutil/utils/fixes.py
@@ -14,6 +14,7 @@
 import pandas as pd
 import sklearn
 import sys
+from types import NoneType
 from abc import ABCMeta, abstractmethod
 from sklearn.base import BaseEstimator, MetaEstimatorMixin, is_classifier, clone
 from sklearn.externals import six
@@ -656,7 +657,7 @@ def _fit(self, X, y, parameter_iterable):
 
         # for debugging
         assert isinstance(X, np.ndarray)
-        assert isinstance(y, (np.ndarray, None))
+        assert isinstance(y, (np.ndarray, NoneType))
 
         # begin sklearn code
         estimator = self.estimator