login form error when change db to mongodb

[Scorpion]

Hey Guys.

I is newbie developer web with web2py. I change db DAL to

db = DAL('mongodb://localhost:27017/apps')

and when i register with auth libary (http://127.0.0.1:8000/welcome/default/user/register?_next=/welcome/default/index) 

the error <type 'exceptions.SyntaxError'> invalid select attribute: for_update

sr for my bad english :D