Inviting friends to my app from c# codebehind

Question

I know how to invite friends to my app using the following javascript:

function sendRequestToManyRecipients() {
    FB.ui({method: 'apprequests',
           message: 'My Great Request'
    }, requestCallback);
  }

I'm using the following c# code

var fb = new FacebookClient(FacebookSettings.AppId, FacebookSettings.AppSecret);
dynamic parameters = new ExpandoObject();
parameters.to = string.Join(",", friendIDs);
parameters.message = "test vanuit codebehind";
parameters.to = [USERID];

try
{
    dynamic result = fb.Post("/apprequests", parameters);
}
catch (FacebookApiException ex)
{
    string msg = ex.Message;
}

But I get the following error: "(GraphMethodException) Unsupported post request."

Does anybody know what I'm doing wrong?

EDIT: Im'm using the C# Facebook SDK 5.3.2.0