error: enableEventValidation="true"/

I have a combo box in my program that has got onchange method too.when I choose some of it's attributes I recieve this error: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.what should I do? and what is this error for? ...