Clicking back button after a Fragment transaction using addToBackStack does nothing

Clicking back button after a Fragment transaction using addToBackStack
does nothing

I want to be able to reverse a replace FragmentTransaction by using
addToBackStack():
FragmentTransaction fragmentTransaction =
getActivity().getSupportFragmentManager().beginTransaction();
Fragment scheduleFragment = new ScheduleFragment();
fragmentTransaction.replace(R.id.content_container, scheduleFragment,
"scheduleFragment");
fragmentTransaction.addToBackStack("scheduleFragment");
fragmentTransaction.commit();
but after that, clicking the back button does nothing.
From the doc and it's supposed to reverse the transaction.
What am I missing?