diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 3644c17c6b3..d6aab2a5fce 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -878,6 +878,9 @@ TLS Upgrade
    object only because the coder caches *protocol*-side data and sporadically
    exchanges extra TLS session packets with *transport*.
 
+   In some situations (e.g. when the passed transport is already closing) this
+   may return ``None``.
+
    Parameters:
 
    * *transport* and *protocol* instances that methods like