diff --git a/Doc/library/inspect.rst b/Doc/library/inspect.rst
index bea12c9..ac89850 100644
--- a/Doc/library/inspect.rst
+++ b/Doc/library/inspect.rst
@@ -567,6 +567,11 @@
 
    Return the frame object for the caller's stack frame.
 
+   This function relies on Python stack frame support in the interpreter, which
+   isn't guaranteed to exist in all implementations of Python. If running in
+   an implmentation without Python stack frame support this function returns
+   ``None``.
+
 
 .. function:: stack([context])
 
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 0005792..7a3f642 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -417,7 +417,8 @@
    that is deeper than the call stack, :exc:`ValueError` is raised.  The default
    for *depth* is zero, returning the frame at the top of the call stack.
 
-   This function should be used for internal and specialized purposes only.
+   This function should be used for internal and specialized purposes only. It
+   is not guaranteed to exist in all implementations of Python.
 
 
 .. function:: getprofile()