Reflected code:

        public static IEnumerable<SearchResults<TResult>> GetResult<TResult>(this IMultiSearch<TResult> multiSearch)
        {
            multiSearch.ValidateNotNullArgument("multiSearch");
            List<Tuple<SearchContext, ProjectionHelper>> searchRequests = new List<Tuple<SearchContext, ProjectionHelper>>();
            foreach (ISearch<TResult> search in multiSearch.Searches)
            {
                SearchContext searchContext = new SearchContext();
                search.ApplyActions(searchContext);
                if (searchContext.Projections.Count == 0)
                {
                    throw new NotSupportedException("MultiSearch does not support search queries without a projection. Please use the MultiSearch<>.Search() extensions when registering individual queries");
                }
                ProjectionHelper helper = new ProjectionHelper(search.Client.Conventions, searchContext.Projections, searchContext.Language, search.Client.CustomizeSerializer);
                foreach (Expression expression in searchContext.Projections.Values)
                {
                    helper.ApplySelector(expression, searchContext.RequestBody, searchContext.Language);
                }
                searchRequests.Add(new Tuple<SearchContext, ProjectionHelper>(searchContext, helper));
            }
            List<SearchResults<JObject>> list = multiSearch.Client.MultiSearch<JObject>(from x in searchRequests select new Tuple<SearchRequestBody, Action<SearchCommand>>(x.Item1.RequestBody, x.Item1.CommandAction), null).ToList<SearchResults<JObject>>();
            List<SearchResults<TResult>> list2 = new List<SearchResults<TResult>>();
            WaitCallback callBack = null;
            for (int i = 0; i < list.Count; i++)
            {
                SearchResults<TResult> item = new SearchResults<TResult>(searchRequests[i].Item2.GetMappedResult<TResult>(searchRequests[i].Item1.RequestBody, list[i]));
                if (searchRequests[i].Item1.Track != null)
                {
                    TrackContext state = new TrackContext
                    {
                        Query = searchRequests[i].Item1.RequestBody.Query,
                        Result = item
                    };
                    if (callBack == null)
                    {
                        callBack = delegate(object threadContext)
                        {
                            ((ITrackContext)threadContext).TryTrack(searchRequests[i].Item1.Track);
                        };
                    }
                    ThreadPool.QueueUserWorkItem(callBack, state);
                }
                list2.Add(item);
            }
            return list2;
        }

Will throw ArgumentOfRangeException here:

                        callBack = delegate(object threadContext)
                        {
                            ((ITrackContext)threadContext).TryTrack(searchRequests[i].Item1.Track);
                        };