diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index e153bc8..6720f19 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1038,10 +1038,11 @@
    *cmp* specifies a custom comparison function of two arguments (iterable
    elements) which should return a negative, zero or positive number depending on
    whether the first argument is considered smaller than, equal to, or larger than
-   the second argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``
+   the second argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``.  The default
+   value is ``None``.
 
    *key* specifies a function of one argument that is used to extract a comparison
-   key from each list element: ``key=str.lower``
+   key from each list element: ``key=str.lower``.  The default value is ``None``.
 
    *reverse* is a boolean value.  If set to ``True``, then the list elements are
    sorted as if each comparison were reversed.
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index d3c5c63..d7e7d49 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -1408,10 +1408,11 @@
    *cmp* specifies a custom comparison function of two arguments (list items) which
    should return a negative, zero or positive number depending on whether the first
    argument is considered smaller than, equal to, or larger than the second
-   argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``
+   argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``.  The default value
+   is ``None``.
 
    *key* specifies a function of one argument that is used to extract a comparison
-   key from each list element: ``key=str.lower``
+   key from each list element: ``key=str.lower``.  The default value is ``None``.
 
    *reverse* is a boolean value.  If set to ``True``, then the list elements are
    sorted as if each comparison were reversed.