change EnableEventValidation="false" on the top of the page..!! for it to work..but it is not a good method..!!
When the EnableEventValidation property is set to true, ASP.NET validates that a control event originated from the user interface that was rendered by that control. A control registers its events during rendering and then validates the events during postback or callback handling. For example, if a list control includes options numbered 1, 2, or 3 when the page is rendered, and if a postback request is received specifying option number 4, ASP.NET raises an exception. All event-driven controls in ASP.NET use this feature by default. If you write client script that changes a control in the client at run time, you might have to use the RegisterForEventValidation method in order to avoid false event validation errors.