diff --git a/openmc/data/function.py b/openmc/data/function.py
index 88d27ce93..a7397e785 100644
--- a/openmc/data/function.py
+++ b/openmc/data/function.py
@@ -1,7 +1,6 @@
 from abc import ABCMeta, abstractmethod
 from collections import Iterable, Callable
 from numbers import Real, Integral
-from six import with_metaclass
 
 import numpy as np
 
@@ -11,9 +10,11 @@ INTERPOLATION_SCHEME = {1: 'histogram', 2: 'linear-linear', 3: 'linear-log',
                         4: 'log-linear', 5: 'log-log'}
 
 
-class Function1D(with_metaclass(ABCMeta, object)):
+class Function1D(object):
     """A function of one independent variable with HDF5 support."""
 
+    __metaclass__ = ABCMeta
+
     @abstractmethod
     def __call__(self): pass