安卓Service通知栏通知无效：报错

各位大侠好，我写的一个 Service 能运行，并且里面的 Toast 也能显示，但是通知栏却没有反应，真机测试也是如此，请问这是为什么？下面是代码：

public class NofyService extends Service { 

    private final String TAG = "NofyService"; 

    @Nullable  

    @Override  

    public IBinder onBind(Intent intent) { return null;} 

    @Override  

    public void onCreate() {
        Toast.makeText(getApplication(), "service", Toast.LENGTH_SHORT).show();
        Log.v(TAG, "onCreate");
    } 

    @Override  

    public int onStartCommand(Intent intent, int flags, int startId) {         

        Notification notification = new Notification.Builder(getApplicationContext())
                .setContentTitle("事件提醒")
                .setContentText("有事件") 

                //.setVibrate(vibrates)  

                .build();
        notification.flags = Notification.FLAG_AUTO_CANCEL;
        NotificationManager nm = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        nm.notify(1, notification);
        Toast.makeText(getApplication(), "service", Toast.LENGTH_SHORT).show();
        Log.v(TAG, "onStart"); return START_STICKY;
    } 
}