Confirm Popup message for image button

I had a radbutton in my code and a confirm popup message.The message worked fine with the radbutton.My requirement changed and I replaced the radbutton with image button and still need a confirm popup message.I get the following error,

0x800a138f - JavaScript runtime error: Unable to get property 'set_cancel' of undefined or null reference

 <asp:ImageButton ID="savebutton" runat="server" ImageUrl="~/Images/save.png" CausesValidation="true" ValidationGroup="reqvalidations"  OnCommand="savebutton_Click" OnClientClick="ConfirmSave()"  CommandName="Save"   Enabled="true" ToolTip="Save" />

JavaScript,

     function ConfirmSave(sender, args) {
          if (Page_ClientValidate("reqvalidations")) {
                args.set_cancel(!window.confirm("Are you sure you want to save?"));

} }

Any help is appreciated.Thanks!

I changed OnClientClick="return ConfirmSave();" and instead of args.set_cancel I used

    var conf = confirm("Are you sure you want to save this request?");
    if (conf== true)
       return true;
    return false;