返回按钮关闭应用程序，即使使用FragmentTransaction.addToBackStack时（）

无我已经阅读计算器上的其他问题已经能够帮助我的问题。据我所知道的，我是在做正确的一切。

None of the other questions I have read on stackoverflow have been able to help with my problem. As far as I can tell, I am doing everything correctly.

我有一个片段一个主/详细流程。

I have a master/detail flow with fragments.

在创建时的主要活动中，主片段装有下列code：

Upon creation of the main activity, the master fragment is loaded with the following code:

    Fragment frag;
    frag = new MainListFragment();//<-- **the master fragment**

    FragmentManager fm = getFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    transaction.replace(R.id.fragment_container, frag);
    Log.d("My Debug Bitches", "stack:" + fm.getBackStackEntryCount());
    transaction.commit();

主片段有一个的ListView ;点击一个列表项目带来了细节片段，像这样：

The master fragment has a ListView; clicking on a list item brings up the details fragment like so:

@Override
public void onListItemClick(ListView listView, View view, int position, long id) {
    super.onListItemClick(listView, view, position, id);


    FragmentManager fm = getFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    SubListFragment frag = new SubListFragment();//<-- **the detail fragment**

    transaction.replace(R.id.fragment_container, frag);
    transaction.addToBackStack(null);
    transaction.commit();
    fm.executePendingTransactions();
    Log.d("My Debug Bitches", "stack:" + fm.getBackStackEntryCount());

}

现在，根据LogCat中，在 BackStackEntryCount 从0变为1 后，我从主片段导航细节片段：

Now, according to LogCat, the BackStackEntryCount changes from 0 to 1 after I navigate from master fragment to detail fragment:

那么，为什么，当我点击后退按钮，同时在细节的片段，该应用程序关闭，而不是返回到主片段??????????

So why is it that, when I click the back button while in the details fragment, that the app closes instead of returning to the master fragment??????????