FragmentActivity: Cannot cast from Fragment to derived class

I am trying to use a Fragment in a FragmentActivity like so:

TutViewerFragment viewer = (TutViewerFragment)    
getSupportFragmentManager().findFragmentById(R.id.tutview_fragment);

And my TutViewerFragment extends Fragment. However I am getting an error:

Cannot cast from Fragment to TutViewerFragment.

I really don't understand why this is happening. Why can't it be cast?