SoapException in WSE 3.0 Webservice

Question

I have this code and I'm getting - SoapException was unhandled: no such operation 'string'. What can be wrong? How to handle such exception? If I put it in try/catch block it doesn't help me a lot.

Code:

    public string PaymentOperation(string amount)
    {
            Microsoft.Web.Services3.SoapEnvelope results = base.SendRequestResponse("PaymentOperation", amount);
            return ((string)(results.GetBodyObject(typeof(string), this.SoapServiceAttribute.TargetNamespace)));     
    }