Membership.CreateUser() error

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

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