[Solved] How do I remove the error Unknown server tag 'ajax:modelpopupextender ?

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VB

ASP.NET

Am trying to add modelpopExtender control on form and also added reference tag
for registring tag prefix as a 'ajax' but still I get error . What will be the problem in the code below.

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="ajax" %>
<ajax:ModalPopupExtender ID="mpeUser" runat="server"
        TargetControlID="hfUserID"
        PopupControlID="pnlAddPopup"
        BehaviorID="mpeUserBehavior"
        DropShadow="true"
        CancelControlID="imgClose"
        PopupDragHandleControlID="popupheader"  />