We were using the .OrderBy extension method with a custom Expression. The Mongo driver seems to strip out the `Convert(...)` that the C# compiler adds (to box value types to reference types). i.e.

Expression<Func<TModel, DateTime>> sortExpression = m => m.UploadDate;
var query = collection.AsQueryable().OrderBy(sortExpression);
query.ToList(); // <-- fails

Pull request is following, on Github.