error after postback

HI all,
 I have a simple web service that return states by country ID. And 2 DropDownList on the page, when i choose country then web service return states according country ID, and bind this states to DropDownList. Then i click Button that cause postback, i get error something like "Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Incorrect postback or callback argument .  Validate event enabled with  <pages enableEventValidation="true"/>" 
Does anybody know how to solve this pproblem ? 
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