i get false value of checkboxes

public CheckBox[] cb = null;
int z=0;
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
        {
        cb = new CheckBox[count - k];
        //Database code.........
        while (dr.Read())//read data from access database
                {
            cb[z] = new CheckBox();
                    cb[z].Text = dr["Member_Name"].ToString();
                        Panel2.Controls.Add(cb[z]);
                        Panel2.Controls.Add(new LiteralControl("</br>"));
                        z = z + 1;
        }
    }
}
protected void Button6_Click(object sender, EventArgs e)
{
        for (int x = 0; x < cb.Length; x++)//ERROR IS HERE:Object reference not set to an instance of an object.
        {
        if (cb[x].Checked == true)//EVERY TIME I GOT FALSE VALUE RATHER I CHECKED BOXES
                {
                //processing check boxes    
        }
    }
}