public async Task AttemptingToAppendANullSequenceToANonNullSequenceThrowsArgumentNullException() { var source = new CelestialBody[0].ToAsyncEnumerable(); IEnumerable <CelestialBody> additional = null; source.ShouldNotBeNull(); additional.ShouldBeNull(); await source.AppendAsync(additional) .ConsumeAsync() .ShouldThrowAsync <ArgumentNullException>(); }