diff --git a/tests/extensions/test_timestamps.py b/tests/extensions/test_timestamps.py
index 91084d0d6..7f484e477 100644
--- a/tests/extensions/test_timestamps.py
+++ b/tests/extensions/test_timestamps.py
@@ -50,7 +50,7 @@ def test_apply(self):
             self.assertIsNone(d)
 
         for p in ('expires', 'unpublished'):
-            self.assertIsNone(item.properties[p])
+            self.assertNotIn(p, item.properties)
 
     def test_validate_timestamps(self):
         item = pystac.read_file(self.example_uri)