diff --git a/requests/models.py b/requests/models.py
index fe85a833..898ed66a 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -386,8 +386,6 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         self.body = body
 
     def prepare_content_length(self, body):
-        if self.headers.get('Content-Length'):
-            return
         if hasattr(body, 'seek') and hasattr(body, 'tell'):
             body.seek(0, 2)
             self.headers['Content-Length'] = str(body.tell())