diff --git a/openmc/material.py b/openmc/material.py
index 17283d4dd..97c7cedca 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -281,7 +281,7 @@ class Material(object):
                   'macroscopic data-set has already been added'.format(self._id)
             raise ValueError(msg)
 
-        if not isinstance(nuclide, (openmc.Nuclide, str)):
+        if not isinstance(nuclide, (openmc.Nuclide, basestring)):
             msg = 'Unable to add a Nuclide to Material ID="{0}" with a ' \
                   'non-Nuclide value "{1}"'.format(self._id, nuclide)
             raise ValueError(msg)
@@ -401,7 +401,7 @@ class Material(object):
                   'macroscopic data-set has already been added'.format(self._id)
             raise ValueError(msg)
 
-        if not isinstance(element, (openmc.Element, str)):
+        if not isinstance(element, (openmc.Element, basestring)):
             msg = 'Unable to add an Element to Material ID="{0}" with a ' \
                   'non-Element value "{1}"'.format(self._id, element)
             raise ValueError(msg)