dropdown selectedIndexChanged and ajax PopUp

Hi all,
I want to out  ajax modal pop up window when I choose "shipment" in my dropdown .But what my problem is  pop up window appear as soon as I click my dropdown .
here is my code.
 <ajax:ModalPopupExtender ID="ModalPopupShippment" runat="server"             Enabled="True" RepositionMode="RepositionOnWindowScroll" TargetControlID="ddlOrder" CancelControlID="lbtnClose" PopupControlID="shipPanel">  </ajax:ModalPopupExtender>
protected void ddlOrder_SelectedIndexChanged1(object sender, EventArgs e)         {             ModalPopupShippment.Hide();             string value = ddlOrder.SelectedValue;   &nb ...