when checkbox selected autopost not working

<div style="width:600px;" > <asp:CheckBox ID="cbUpgrade" CssClass="cbUpgrade" runat="server" AutoPostBack="true" />&nbsp;I would like to upgrade my print and online catalogue listing to enhanced (at a cost of $250):&nbsp; </div>
<div id="CatalogueAdDiv" style="display:none;" runat="server">
test here</div>
</div>
default.aspx.cs
protected void cbUpgrade_CheckedChanged(object sender, EventArgs e) { if (cbUpgrade.Checked) CatalogueAdDiv.Visible = true;
}