diff --git a/jupyterhub/tests/test_app.py b/jupyterhub/tests/test_app.py
index accdd430..a918e9b7 100644
--- a/jupyterhub/tests/test_app.py
+++ b/jupyterhub/tests/test_app.py
@@ -199,6 +199,18 @@ def test_cookie_secret_env(tmpdir, request):
     assert not os.path.exists(hub.cookie_secret_file)
 
 
+def test_cookie_secret_string_():
+    cfg = Config()
+
+    cfg.JupyterHub.cookie_secret = "not hex"
+    with pytest.raises(ValueError):
+        JupyterHub(config=cfg)
+
+    cfg.JupyterHub.cookie_secret = "abc123"
+    app = JupyterHub(config=cfg)
+    assert app.cookie_secret == binascii.a2b_hex('abc123')
+
+
 async def test_load_groups(tmpdir, request):
     to_load = {
         'blue': ['cyclops', 'rogue', 'wolverine'],