Zenfolio API GetChallenge function giving no response .NET

I am using zenfolio api v 1.8 in my .NET project to get photos and other data from zenfolio. its working correctly on my live server. however when i am trying to run the zenfolio api on my pc (windows), in debug mode its giving error in the GetChallenge function. the error its giving is:

The request failed with an empty response.
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at xxx.ZenfolioClient.ZfApiRef.ZfApi.GetChallenge(String loginName)

and here is the getChallenge function:

[System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://www.zenfolio.com/api/1.8/GetChallenge", RequestNamespace="http://www.zenfolio.com/api/1.8", ResponseNamespace="http://www.zenfolio.com/api/1.8", Use=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)]
        public AuthChallenge GetChallenge(string loginName) {
            object[] results = this.Invoke("GetChallenge", new object[] {
                        loginName});
            return ((AuthChallenge)(results[0]));
        }

What i have checked through breakpoints is that its bringing the correct value in the loginName parameter in GetChallenge function but the Invoke function inside it is not able to bring any response or is bringing wrong response as the breakpoint after that line is not working and the error is appearing .. Any help