C# - OnClick Event Not Firing On ModalDialog?

I have a Modal Dialog:

function ShowPopup()
{
window.showModalDialog('dialog.aspx', null, 'status:no;dialogWidth:950px;dialogHeight:150 px');
}

Then its called in the code behind

Page.ClientScript.RegisterStartupScript(this.GetType(), "popUpScript", "ShowPopup();", true);

The dialog.aspx has two buttons:

<asp:Button id="btn1" runat="server" Text="Button 1" OnClick="btn1_Click"></asp:Button>
<asp:Button id="btn2" runat="server" Text="Button 2" OnClick="btn2_Click"></asp:Button>

However, the Click events in the code behind are never getting fired.

protected void Page_Load(object sender, EventArgs e)
{
}
protected void btn1_Click(object sender, System.EventArgs e)
{
Response.Redirect(url)
}
protected void btn2_Click(object sender, System.EventArgs e)
{
Response.Redirect(url);
}
}