PreferenceManager cannot load default value of custom preference

Question

I used code sample for TimePreference from link. Related to my question methods:

@Override
protected Object onGetDefaultValue(TypedArray a, int index) {
    Log.v(TAG, "a.getString(index):  " + a.getString(index));
    Log.v(TAG, "hasKey(): " + hasKey() );
    return a.getString(index);
}

@Override
protected void onSetInitialValue(boolean restoreValue, Object defaultValue) {
    String time=null;

    if (restoreValue) {
        if (defaultValue==null) {
            time=getPersistedString(DEFAULT_NOTIF_TIME_STRING);
        }
        else {
            time=getPersistedString(defaultValue.toString());
        }
    }
    else {
        time=defaultValue.toString();
    }

    lastHour=getHour(time);
    lastMinute=getMinute(time);
}

In notification_time_fragment.xml:

<mypackage.preferences.TimePreference
    android:key ="@string/notif_time_key"
    android:title="@string/notif_time_title"
    android:defaultValue="08:00"
    android:persistent="true"
    />

In activity

PreferenceManager.setDefaultValues(this, R.xml.notification_time_fragment, true);
SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);
String notificationTime = sharedPreferences.getString(getString(R.string.notif_time_key), "can't read time");

And notificationTime is equal to "can't read time". Logging from onGetDefaultValue:

a.getString(index):  08:00
hasKey(): false

I guess that this false from hasKey() is the reason why the preference is not saved in sharedPreference by command PreferenceManager.setDefaultValues().

I must say that android:defaultValue of EditTextPreference, CheckBoxPreference are properly retrieved from xml-resource using the same commands.

Why hasKey() is false? What is wrong in TimePreference? How can I make it to write its default value without launching PreferenceActivity and without "dirty coding" in MainActivity?