Handle non-iterable arguments reasonable
diff --git a/tests/test_fernet.py b/tests/test_fernet.py
index 91af32a..58f89cb 100644
--- a/tests/test_fernet.py
+++ b/tests/test_fernet.py
@@ -146,3 +146,7 @@
     def test_no_fernets(self, backend):
         with pytest.raises(ValueError):
             MultiFernet([])
+
+    def test_non_iterable_argument(self, backend):
+        with pytest.raises(TypeError):
+            MultiFernet(None)