Button's OnClick event doesn't fire ?

[M-Dayyan]

Hi friends,
The following code is my ASP.NET page :

<MdsMenu:MenuItem Text="Button" Href="#">
<asp:Button ID="Button1" runat="server" Text="Button 01" OnClick="Button_Click" />
<asp:Button ID="Button2" runat="server" Text="Button 02" OnClick="Button_Click" />
</MdsMenu:MenuItem>

I don't know why Button_Click event doesn't fire !!!

protected void Button_Click(object sender, EventArgs e)
{
Button senderButton = sender as Button;
Label1.Text = "شما دکمه " + senderButton.ID.ToString() + " را فشار دادید";
}

MdsMenu:MenuItem is my custom WebControl.
If I insert a Button control out of MdsMenu:MenuItem tag it works well but if I inserted it within MdsMenu:MenuItem tag it doesn't fire the concerned method.
What's wrong with my code ?

Thanks