hi i am new to umbraco 

ive recent made a login and signup page and use umbraco.cms.businesslogic.member.Member

login is working prefectly fine

but during registeration it gives error while checking for the previous existed users

here is the Register.aspx.cs code

{

if (string.IsNullOrWhiteSpace(tbEmail.Text) | string.IsNullOrWhiteSpace(tbPassword.Text) | string.IsNullOrWhiteSpace(tbUsername.Text))

            {

                litError.Text = string.Format(ErrorFormat, "Please complete all fields");

                return;

            }

 

            // Check the user isn't already registered

            if ((Member.GetMemberFromEmail(tbEmail.Text) == null) && (Member.GetMemberFromLoginName(tbUsername.Text) == null))

            {

                // Set the member type and group

                var mt = MemberType.GetByAlias(MembersType);

                var addToMemberGroup = MemberGroup.GetByName(MembersGroup);

 

                //create the member, and set the password and email

                var m = Member.MakeNew(tbUsername.Text, mt, new User(0));

                m.Password = tbPassword.Text;

                m.Email = tbEmail.Text;

 

                // Add the member to the group

                m.AddGroup(addToMemberGroup.Id);

 

                //Save member

                m.Save();

 

                //Generate member Xml Cache

                m.XmlGenerate(new System.Xml.XmlDocument());

 

                // NOTE: This Is Optional

                // Login the user

                FormsAuthentication.SetAuthCookie(tbUsername.Text, false);

 

                // Redirect to successful page (Usually their profile or member page)

                Response.Redirect(library.NiceUrl(SuccessfulLoginPage));

            }

            else

            {

                // Error, member already exists with email or username used

                litError.Text = string.Format(ErrorFormat, "User already exists");

            }

}

it gives error on line that ive made BOLD