Android – findFragmentById always returns null

I'm defining an ID for my fragment in the xml layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/test_fragment"
...

Then I add this fragment in the activity's onCreate method:

MyFragment myFragment = new MyFragment();
fragmentTransaction.add(R.id.fragment_container, myFragment);
fragmentTransaction.commit();

This is all working fine. Replacing fragments and is also working.

Later I'm trying to retrieve this fragment by its ID in one of the activity's methods:

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentById(R.id.test_fragment);

Doing so leads to myFragment being null. Always.

When I try to do the same with tags instead of IDs I can retrieve the fragment by its tag without any problems:

MyFragment myFragment = new MyFragment();
fragmentTransaction.add(R.id.fragment_container, myFragment, "testfragment");
fragmentTransaction.commit();

…

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentByTag("testfragment");

Why can't findFragmentById find the fragment, but findFragmentByTag does so? Am I missing something?