The following error is returned When making the request to http://localhost/contacts.

{"Message":"The request is invalid.","MessageDetail":"The parameters dictionary contains an invalid entry for parameter 'groupId' for method 'System.String Get(System.Nullable1[System.Int32])' in 'SelfHost.ContactsController'. The dictionary contains a value of type 'System.Collections.Generic.List1[System.Int32]', but the parameter requires a value of type 'System.Nullable`1[System.Int32]'."}

Assuming the route is:
"{controller}/{groupId} where groupId is optional

and the controller looks like this:
public class ContactsController
{

public HttpResponseMessage Get(int? groupId = null){}

}