diff --git a/jupyter_client/tests/test_jsonutil.py b/jupyter_client/tests/test_jsonutil.py
index bc3311052..ef739b936 100644
--- a/jupyter_client/tests/test_jsonutil.py
+++ b/jupyter_client/tests/test_jsonutil.py
@@ -109,6 +109,7 @@ def test_json_default():
         (True, None),
         (False, None),
         (None, None),
+        ({"key": b"\xFF"}, {"key": "/w==\n"}),
         # Containers
         ([1, 2], None),
         ((1, 2), [1, 2]),