Django flatpages app will not work

django 在安装 flatpages的时候，遇到问题
安装步骤是修改settings文件：
1）在settings.py里添加'django.contrib.flatpages' 到 INSTALLED_APPS
2）把 'django.contrib.flatpages.middleware.FlatpageFallbackMiddleware' 添加到 MIDDLEWARE_CLASSES
3）找到当前调试用的域名，修改settings.py里SITE_ID的值
4）然后python manage.py syncdb

但是报错：

CommandError: One or more models did not validate:

flatpages.flatpage: 'sites' has an m2m relation with model: , which has either not been installed or is abstract.

---

自问自答，
在 INSTALLED_APPS的django.contrib.flatpages前面添加 django.contrib.sites