ASp.net Radio Buttons list selected Index always Zero

I have Radio Buttons list with 4 items in it and one button object and I use this Code in vb.net in the code behind

   Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
    Session("k1") = RadioButtonList1.SelectedIndex.ToString()
    Label1.Text = Session("k1").ToString()
End Sub

I want to get the the value of the select index of selected item when button clicked but it's always give me zero ??? Any one can help Thanks.