c# - how to post like with facebook c# sdk

Question

How do you post a like to a facebook app using the c# sdk ? I tried with this code:

...
FacebookClient app = new FacebookClient(accessToken);
Dictionary<string,object> parameters = new Dictionary<string, object>();
app.PostAsync(string.Format("{0}/likes", appId), parameters);
...

but I get ArgumentNullException: stream cannot be null