This is how I call a service with .NET:
var requestedURL = \"https://accounts.google.com/o/oauth2/token?code=\" + code + \"&client_id=\" + client_id + \"
I had the same error when I imported web requests from fiddler captured sessions to Visual Studio webtests. Some POST requests did not have a StringHttpBody tag. I added an empty one to them and the error was gone. Add this after the Headers tag: