Get value from javascript function into Dropdown and Display selected Value

Question

Hello Friends,

I've two dropdownlist(ddlCountry,ddlState). Onchange event of CountryDropdown(ddlCountry) StateNames are displayed into StateName dropdown using javascript. And it's working fine but prooblem is if i print the value of selected StateName on a button click at code-behind i got an error <b>Object reference set to an instance of an object</b>. So can anybody let me know what should i do know
   bah10 z