diff --git a/src/tests/observable_test.py b/src/tests/observable_test.py
index 3c5a8897..9f1a5367 100644
--- a/src/tests/observable_test.py
+++ b/src/tests/observable_test.py
@@ -745,7 +745,14 @@ def test_time_buffer_read_until_closed(self):
 
     def test_time_buffer_aggregated_read_until_closed(self):
         observable = self.create_observable()
-        buffered_observable = observable.time_buffered(100, lambda chunks: ['|'.join(chunks)])
+        # Use 30ms period (not 100ms) to avoid a timing race: the helper
+        # _test_read_until_closed sends late messages after 100ms sleep.
+        # With period=100ms both the flush thread and the async thread fire at
+        # ~t=100ms; the flush thread can close the buffered observable before
+        # read_until_closed subscribes (at t≈140ms), resulting in empty data.
+        # With period=30ms flush cycles land at t=30/60/90/120ms; the subscriber
+        # is set up at t≈49ms and the late-message flush happens at t≈120ms.
+        buffered_observable = observable.time_buffered(30, lambda chunks: ['|'.join(chunks)])
 
         data, _, late_messages = self._test_read_until_closed(
             observable,