diff --git a/Doc/library/gc.rst b/Doc/library/gc.rst
index 6d5c64df1a1..0961ca4aaa9 100644
--- a/Doc/library/gc.rst
+++ b/Doc/library/gc.rst
@@ -50,6 +50,9 @@ The :mod:`gc` module provides the following functions:
    is run.  Not all items in some free lists may be freed due to the
    particular implementation, in particular :class:`float`.
 
+   The effect of calling ``gc.collect()`` while the interpreter is already
+   performing a collection is undefined.
+
 
 .. function:: set_debug(flags)