使用Flask 0.10.1时，获取'TypeError：ObjectId（''）不是JSON可序列化的'

I forked the Flask example, Minitwit, to work with MongoDB and it was working fine on Flask 0.9, but after upgrading to 0.10.1 I get the error in title when I login when I try to set the session id.I forked the Flask example, Minitwit, to work w