diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 22397f2..871cec7 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -393,13 +393,20 @@
    :func:`setrecursionlimit`.
 
 
-.. function:: getsizeof(object)
+.. function:: getsizeof(object[, default])
 
    Return the size of an object in bytes. The object can be any type of
    object. All built-in objects will return correct results, but this
-   does not have to hold true for third-party extensions as it is implementation 
+   does not have to hold true for third-party extensions as it is implementation
    specific.
 
+   The *default* argument allows to define a value which will be returned
+   if the object type does not provide means to retrieve the size and would
+   cause a `TypeError`. 
+
+   func:`getsizeof` calls the object's __sizeof__ method and adds an additional
+   garbage collector overhead if the object is managed by the garbage collector.
+
    .. versionadded:: 2.6