diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index db5fae2006678a..7ecad466c80c9b 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -1052,6 +1052,10 @@ is used when no command-line argument was present::
    >>> parser.parse_args([])
    Namespace(foo=42)
 
+Because ``nargs='*'`` gathers any supplied values into a list, an absent
+positional argument yields an empty list (``[]``). Only a non-``None``
+*default* overrides this (so ``default=None`` still gives ``[]``).
+
 For required_ arguments, the ``default`` value is ignored. For example, this
 applies to positional arguments with nargs_ values other than ``?`` or ``*``,
 or optional arguments marked as ``required=True``.