diff --git a/requests/models.py b/requests/models.py
index ae6eb708..ec2edc20 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -273,7 +273,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
     """The fully mutable :class:`PreparedRequest <PreparedRequest>` object,
     containing the exact bytes that will be sent to the server.
 
-    Generated from either a :class:`Request <Request>` object or manually.
+    Instances are generated from a :class:`Request <Request>` object, and
+    should not be instantiated manually; doing so may produce undesirable
+    effects.
 
     Usage::