diff --git a/Doc/library/urllib.request.rst b/Doc/library/urllib.request.rst
index 7a0ccfd024beb3..2ba3f1f7afd1f9 100644
--- a/Doc/library/urllib.request.rst
+++ b/Doc/library/urllib.request.rst
@@ -252,6 +252,11 @@ The following classes are provided:
    bytes objects, while ``Transfer-Encoding: chunked`` as specified in
    :rfc:`7230`, Section 3.3.1 will be used to send files and other iterables.
 
+   *data* is not serialized before it is sent: the elements of an iterable
+   are sent as is, so a :class:`dict` yields only its keys.  Structured
+   data such as a JSON object must be encoded to bytes first, for example
+   with :func:`json.dumps`.
+
    For an HTTP POST request method, *data* should be a buffer in the
    standard :mimetype:`application/x-www-form-urlencoded` format.  The
    :func:`urllib.parse.urlencode` function takes a mapping or sequence