code is not working with modal popup

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ASP.NET

protected void RbListYesNo_SelectedIndexChanged(object sender, EventArgs e)
       {
          
          if (RbListYesNo.SelectedIndex ==0)
           {
              ModalPopupExtender1.Hide();
           }
           else if(RbListYesNo.SelectedIndex==1)
           {
               txtFlwId.Text = "";
               ModalPopupExtender1.Hide();
           }
       }

this is the code on modal popup.
as soon it comes to the" if condition" it not going inside the loop if the SelectedIndex is 0.
i have made the radio button list as auto post back =true.

plz help me..