a problem when i create a membershipUser

the codes listed below is what i wrote to understand the membership in asp.net 2.0.But when i run it in debug mode,it stopped with the error message "Object reference not set to an instance of an object.  " i can make out that the mu object is null but did not know why. Somebody would please give me some advice?i will appreciate it so much!!!     

   protected void Button1_Click(object sender, EventArgs e)
        {
            MembershipCreateStatus ms;
            MembershipUser mu = Membership.CreateUser("blik", "blik", blikbosyang@gmail.com, "who are you?", "blik", true, out ms);
            ...