getSupportFragmentManager().beginTransaction().add(android.R.id.content, f) not working

getSupportFragmentManager().beginTransaction().add(android.R.id.content,
f) not working

// Place an ArticleFragment as our content pane
final ArticleFragment f = new ArticleFragment();
getSupportFragmentManager().beginTransaction().add(android.R.id.content,
f).commit();This lines of code are from the NewsReader sample app
Why they are not working if the activity extends ActionBarActivity?
Everything works fine if the activity extends FragmentActivity.
Why?
Ralph