Object reference not set to an instance of an object.-i got null value of checkbox array

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.
{
//processing check boxes
}
}