diff --git a/flask/helpers.py b/flask/helpers.py
index 418c4b17..ec513d8f 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -166,7 +166,8 @@ def url_for(endpoint, **values):
     ==================== ======================= =============================
 
     Variable arguments that are unknown to the target endpoint are appended
-    to the generated URL as query arguments.
+    to the generated URL as query arguments.  If the value of a query argument
+    is `None`, the whole pair is skipped.
 
     For more information, head over to the :ref:`Quickstart <url-building>`.