diff --git a/moto/swf/models/__init__.py b/moto/swf/models/__init__.py
index 18db47e2..8702bc37 100644
--- a/moto/swf/models/__init__.py
+++ b/moto/swf/models/__init__.py
@@ -1,4 +1,5 @@
 from __future__ import unicode_literals
+import six
 
 import boto.swf
 
@@ -47,7 +48,7 @@ class SWFBackend(BaseBackend):
             self._check_string(parameter)
 
     def _check_string(self, parameter):
-        if not isinstance(parameter, basestring):
+        if not isinstance(parameter, six.string_types):
             raise SWFSerializationException(parameter)
 
     def _check_none_or_list_of_strings(self, parameter):
@@ -58,7 +59,7 @@ class SWFBackend(BaseBackend):
         if not isinstance(parameter, list):
             raise SWFSerializationException(parameter)
         for i in parameter:
-            if not isinstance(i, basestring):
+            if not isinstance(i, six.string_types):
                 raise SWFSerializationException(parameter)
 
     def _process_timeouts(self):