Adding New Fields To Create User Wizard?

i m trying to add new field named mobile in my create user wizard control for that in my web.config i add

profile
enabled ="true">properties>add
name ="Mobile"
type ="string"/>properties>profile>

in my create user wizard using edit template i add one textbox named 'txtmobile'

nd in my cs i write

CreateUserWizard2_CreatedUser
{
ProfileCommon p = (ProfileCommon)ProfileCommon.Create(CreateUserWizard2.UserName,
true);
TextBox txtM = ((TextBox)CreateUserWizard2.CreateUserStep.ContentTemplateContainer.FindControl("txtMobile"));
p.Mobile = txtM.Text;
//p.Mobile = ((TextBox)CreateUserWizard2.CreateUserStep.ContentTemplateContainer.FindControl("txtMobile")).Text;
p.Save();
}

but it throws the error of 'object reference' nd also let me know how to store that field in db?