Error : Cannot convert type 'string' to 'net.webservicex.www.Currency' .

Question

Error : Cannot convert type 'string' to 'net.webservicex.www.Currency' .

error comes from:

net.webservicex.www.CurrencyConvertor conver = new net.webservicex.www.CurrencyConvertor();       

        double d = conver.ConversionRate((Currency)ddfr.SelectedValue,(Currency)ddto.SelectedValue);


ddfr and ddto are dropdownlist.