ChangeEvents.getLatestChangeLogToken returns null in Atompub binding

Our test code is like
ChangeEvents changeEvents = session.getContentChanges(latestChangeLogToken, false, 25;

String newlatestChangeLogToken = changeEvents.getLatestChangeLogToken();
assertNotNull(newlatestChangeLogToken);

But ChangeEvents.getLatestChangeLogToken returns null. I looked at the response feed. I cannot figure out which part is missing. Can you tell from which field OpenCMIS Client API reads the latestChangeLogToken in the feed?