diff --git a/docs/topics/http/sessions.txt b/docs/topics/http/sessions.txt
index 1e3970e9d2..7366ea2d6d 100644
--- a/docs/topics/http/sessions.txt
+++ b/docs/topics/http/sessions.txt
@@ -348,6 +348,9 @@ Bundled serializers
         >>> request.session['0']
         'bar'
 
+    Similarly, data that can't be encoded in JSON, such as non-UTF8 bytes like
+    ``'\xd9'`` (which raises :exc:`UnicodeDecodeError`), can't be stored.
+
     See the :ref:`custom-serializers` section for more details on limitations
     of JSON serialization.