diff --git a/Doc/library/urllib.request.rst b/Doc/library/urllib.request.rst
index 95e4d2627c8b23..5c8333d3dc8a7d 100644
--- a/Doc/library/urllib.request.rst
+++ b/Doc/library/urllib.request.rst
@@ -1050,6 +1050,10 @@ AbstractBasicAuthHandler Objects
    authenticate for, *req* should be the (failed) :class:`Request` object, and
    *headers* should be the error headers.
 
+   *headers* must be a mapping-like object with case-insensitive lookup
+   that implements the ``get_all()`` method,
+   such as :class:`email.message.Message` or :class:`wsgiref.headers.Headers`.
+
    *host* is either an authority (e.g. ``"python.org"``) or a URL containing an
    authority component (e.g. ``"https://python.org/"``). In either case, the
    authority must not contain a userinfo component (so, ``"python.org"`` and
@@ -1091,6 +1095,9 @@ AbstractDigestAuthHandler Objects
    should be the (failed) :class:`Request` object, and *headers* should be the
    error headers.
 
+   *headers* must be a mapping-like object with case-insensitive lookup,
+   such as :class:`email.message.Message` or :class:`wsgiref.headers.Headers`.
+
 
 .. _http-digest-auth-handler: