C# asp.net Membership.Getuser() always gets null

Question

i have the folowing code in activation page after the user activate the account with email link. i am getting "object reference not set to an instance of an object" error for the line " newUser.IsApproved = true;". i check userId string and it gets the user Id string and the string is not empty.Membership.Getuser() always gets null , anyone can help me?

    //request for signed user UserId
    Guid activationCode = new Guid(Request.QueryString["ActivationCode"]);
    string userId = "";

    string ConnectionString = ConfigurationManager.AppSettings["myConnectionString"];
    try
    {
        using (SqlConnection sqlConnection = new SqlConnection(ConnectionString))
        {
            SqlCommand sqlCommand = new SqlCommand("SELECT user_id FROM ActivationCode WHERE activation_code LIKE '%" + activationCode + "%'", sqlConnection);
            sqlConnection.Open();
            SqlDataReader userIdRdr;
            userIdRdr = sqlCommand.ExecuteReader();
            while (userIdRdr.Read())
            {
                userId = userIdRdr["user_id"].ToString();
            }
            sqlConnection.Close();

            MembershipUser newUser = Membership.GetUser(userId);
            //activate signed user
            newUser.IsApproved = true;
            Membership.UpdateUser(newUser);
        }
    }

Answer 1

got it!!!

Guid guid = new Guid(userId);
MembershipUser newUser = Membership.GetUser(guid);   
newUser.IsApproved = true;
Membership.UpdateUser(newUser);