diff --git a/requests/models.py b/requests/models.py
index 845a778e..a2775550 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -301,7 +301,8 @@ class Request(object):
                 req = _Request(url, data=self._enc_data, method=self.method)
 
         if self.headers:
-            req.headers.update(self.headers)
+            for k,v in self.headers.iteritems():
+                req.add_header(k, v)
 
         if not self.sent or anyway: