GOOGLE SSO using SAML

Hey people,
I am trying to integrate the SAML SSO with my website,
when I send SAML request to my IDP using code below

refer this code
AccountSettings ac = new AccountSettings();
Models.GoogleSamlData.saml.AuthRequest req = new Models.GoogleSamlData.saml.AuthRequest(new AppSetting(), ac);
String url = (ac.idp_sso_target_url + "?SAMLRequest=" +
Server.UrlEncode(req.GetRequest(Models.GoogleSamlData.saml.AuthRequest.AuthRequestFormat.Base64)));
return RedirectPermanent(url);

I Get an error saying

Invalid Request, invalid idpId in request URL, check if SSO URL is configured properly on SP side

but I am sure that idp id is correct,
think the problem is with the format or something else that I cant figure out