diff --git a/rest_framework/authentication.py b/rest_framework/authentication.py
index 0cd2dd0db..23ef49d69 100644
--- a/rest_framework/authentication.py
+++ b/rest_framework/authentication.py
@@ -7,6 +7,7 @@ import base64
 
 from django.contrib.auth import authenticate, get_user_model
 from django.middleware.csrf import CsrfViewMiddleware
+from django.utils.six import string_types
 from django.utils.translation import ugettext_lazy as _
 
 from rest_framework import HTTP_HEADER_ENCODING, exceptions
@@ -19,7 +20,7 @@ def get_authorization_header(request):
     Hide some test client ickyness where the header can be unicode.
     """
     auth = request.META.get('HTTP_AUTHORIZATION', b'')
-    if isinstance(auth, str):
+    if isinstance(auth, string_types):
         # Work around django test client oddness
         auth = auth.encode(HTTP_HEADER_ENCODING)
     return auth