diff --git a/requests/models.py b/requests/models.py
index 104b6db7..8885569a 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -346,7 +346,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         ])
 
         try:
-            length = super_len(data)
+            length = str(super_len(data))
         except (TypeError, AttributeError):
             length = False
 
diff --git a/test_requests.py b/test_requests.py
index 4a709827..425e3570 100644
--- a/test_requests.py
+++ b/test_requests.py
@@ -9,6 +9,12 @@ import unittest
 
 import requests
 from requests.auth import HTTPDigestAuth
+from requests.compat import str
+
+try:
+    import StringIO
+except ImportError:
+    import io as StringIO
 
 HTTPBIN = os.environ.get('HTTPBIN_URL', 'http://httpbin.org/')
 
@@ -131,8 +137,6 @@ class RequestsTestCase(unittest.TestCase):
         self.assertEqual(r.status_code, 200)
 
     def test_BASICAUTH_TUPLE_HTTP_200_OK_GET(self):
-
-
         auth = ('user', 'pass')
         url = httpbin('basic-auth', 'user', 'pass')