I am using asp.net 3.5 framework. My page has modelpopup in that ajax tab controls used. one of the tab has asp:dropdownlist, this dropdown list is binding data from dataset. the tab which has dropdown lists is not working. it is working fine in IE9.0. Sys.WebForms.PageRequestManagerServerErrorException: Sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.