how to verify a single checkbox checked in html form in python using web.py?

Suppose I have a form in html having a check-box unless it is checked u can't go back to next page.

* I accept the Terms and Conditions

I m handling it in the post method like this: -

def POST(self): 
    j=web.input()
    print j
    agreement = j.agrmnt
    if (agreement = 'Agreed'):
       raise web.seeother('/userreg')
    else:
        raise web.seeother('/failed')

Problem is its giving attribute error on 'agrmnt' when i don't check the checkbox.