Membership.CreateUser() Null Exception

I use Membership.CreateUser() method to create user , it works fine when browsing site locally i.e. with in LAN but when I put my site Online and access it from outside my network it gave Null Exception.
Membership.CreateUser("Name", "password" "Email", "Question", "Answer", true, out result);                                MembershipUser u = Membership.GetUser(Name);                 u.Comment = "1";                 Membership.UpdateUser(u);
Kindly help
Thanks