diff --git a/requirements.txt b/requirements.txt
index d781e910..59462dc7 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,3 +3,4 @@ requests==2.2.1
 celery==3.1.11
 gevent==1.0.0
 redis==2.9.1
+six==1.9.0
diff --git a/snowplow_tracker/tracker.py b/snowplow_tracker/tracker.py
index d31439f1..700b9832 100644
--- a/snowplow_tracker/tracker.py
+++ b/snowplow_tracker/tracker.py
@@ -21,6 +21,7 @@
 
 import time
 import uuid
+import six
 from snowplow_tracker import payload, _version
 from snowplow_tracker import subject as _subject
 from contracts import contract, new_contract
@@ -44,9 +45,9 @@
 
 class Tracker:
 
-    new_contract("non_empty_string", lambda s: isinstance(s, str)
+    new_contract("non_empty_string", lambda s: isinstance(s, six.string_types)
                  and len(s) > 0)
-    new_contract("string_or_none", lambda s: (isinstance(s, str)
+    new_contract("string_or_none", lambda s: (isinstance(s, six.string_types)
                  and len(s) > 0) or s is None)
     new_contract("payload", lambda s: isinstance(s, payload.Payload))