diff -r 47394ceeaba1 Doc/library/future_builtins.rst
--- a/Doc/library/future_builtins.rst	Sun Feb 16 14:51:17 2014 -0500
+++ b/Doc/library/future_builtins.rst	Mon Feb 17 10:34:19 2014 +0000
@@ -50,6 +50,11 @@
 
    Works like :func:`itertools.imap`.
 
+   .. note::
+
+      In Python 3, :func:`map` does not accept ``None`` for the
+      function argument.
+
 .. function:: oct(object)
 
    Works like the built-in :func:`oct`, but instead of :meth:`__oct__` it will