diff --git a/requests/models.py b/requests/models.py
index 6d7be5bf..c11858b8 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -325,6 +325,8 @@ class Request(object):
 
         if isinstance(data, bytes):
             return data, data
+        if isinstance(data, str):
+            return data, data
         elif hasattr(data, '__iter__'):
             try:
                 dict(data)