diff --git a/requests/models.py b/requests/models.py
index 14518006..6ed2b599 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -394,7 +394,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         elif body is not None:
             l = super_len(body)
             if l:
-                self.headers['Content-Length'] = super_len(l)
+                self.headers['Content-Length'] = str(l)
         elif self.method not in ('GET', 'HEAD'):
             self.headers['Content-Length'] = '0'
 
diff --git a/requests/sessions.py b/requests/sessions.py
index 26131fc5..961fc3bc 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -429,12 +429,14 @@ class Session(SessionRedirectMixin):
 
     def send(self, request, **kwargs):
         """Send a given PreparedRequest."""
-        # It's possible that users might accidentally send a Request object.
-        # Guard against that specific failure case.
+        # Set defaults that the hooks can utilize to ensure they always have
+        # the correct parameters to reproduce the previous request.
         kwargs.setdefault('stream', False)
         kwargs.setdefault('verify', True)
         kwargs.setdefault('proxies', {})
 
+        # It's possible that users might accidentally send a Request object.
+        # Guard against that specific failure case.
         if getattr(request, 'prepare', None):
             raise ValueError('You can only send PreparedRequests.')