I have a IObservable [named rows in the sample below] from Reactive extensions framework and I want to add index numbers to each object it observes.

I've tried to implement this using Zip function:

rows.Zip(Enumerable.Range(1, int.MaxValue), (row, index) => 
    new { Row = row, Index = index })
    .Subscribe(a => ProcessRow(a.Row, a.Index), () => Completed());

.. but unfortunately this throws

ArgumentOutOfRangeException: Specified argument was out of the range of valid values. Parameter name: disposables

Am I understanding the Zip function wrong or is there a problem with my code?

The Range part of the code doesn't seem to be the problem and the IObservable isn't yet receiving any events.