dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'thirteenth')

Error message

Value cannot be null. (Parameter 'thirteenth')

What it means

The 13-source Qbservable.Zip overload throws ArgumentNullException naming 'thirteenth' when the last source sequence is null. The check at line 1929 is the final guard before building the expression tree; without it, the null would surface as a confusing failure inside CreateQuery or the provider.

Solutions

1. Supply a valid IObservable<TThirteenth> for the thirteenth argument
2. Use Observable.Empty<TThirteenth>() as a null-safe default when the source is absent
3. Fix the producer so it never returns a null sequence

Example fix

// before
var zipped = QbservableEx.Zip(first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth /* null */);
// after
if (thirteenth == null) thirteenth = Observable.Empty<TThirteenth>();
var zipped = QbservableEx.Zip(first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth);

if (thirteenth is null) thirteenth = Observable.Empty<TThirteenth>();

bool hasThirteenth = thirteenth is not null;

try
{
    var zipped = QbservableEx.Zip(first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth);
}
catch (ArgumentNullException ex)
{
    logger.LogError("Zip failed: parameter {Param} was null", ex.ParamName);
    throw;
}

When it happens

Trigger: Calling QbservableEx.Zip(first..thirteenth) with thirteenth == null.

Common situations: Optional trailing source in a programmatically assembled query; null returned by a lookup/factory for the final stream in a 13-way merge.

Related errors

• throw new ArgumentNullException(nameof(first))
• throw new ArgumentNullException(nameof(resultSelector))
• throw new ArgumentNullException(nameof(second))
• throw new ArgumentNullException(nameof(sources))
• Value cannot be null. (Parameter 'fourteenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2ba722555894cc8b. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:1929

                throw new ArgumentNullException(nameof(fourth));
            if (fifth == null)
                throw new ArgumentNullException(nameof(fifth));
            if (sixth == null)
                throw new ArgumentNullException(nameof(sixth));
            if (seventh == null)
                throw new ArgumentNullException(nameof(seventh));
            if (eighth == null)
                throw new ArgumentNullException(nameof(eighth));
            if (ninth == null)
                throw new ArgumentNullException(nameof(ninth));
            if (tenth == null)
                throw new ArgumentNullException(nameof(tenth));
            if (eleventh == null)
                throw new ArgumentNullException(nameof(eleventh));
            if (twelfth == null)
                throw new ArgumentNullException(nameof(twelfth));
            if (thirteenth == null)
                throw new ArgumentNullException(nameof(thirteenth));

            return first.Provider.CreateQuery<(TFirst First, TSecond Second, TThird Third, TFourth Fourth, TFifth Fifth, TSixth Sixth, TSeventh Seventh, TEighth Eighth, TNinth Ninth, TTenth Tenth, TEleventh Eleventh, TTwelfth Twelfth, TThirteenth Thirteenth)>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TFirst>, IObservable<TSecond>, IObservable<TThird>, IObservable<TFourth>, IObservable<TFifth>, IObservable<TSixth>, IObservable<TSeventh>, IObservable<TEighth>, IObservable<TNinth>, IObservable<TTenth>, IObservable<TEleventh>, IObservable<TTwelfth>, IObservable<TThirteenth>, IQbservable<(TFirst First, TSecond Second, TThird Third, TFourth Fourth, TFifth Fifth, TSixth Sixth, TSeventh Seventh, TEighth Eighth, TNinth Ninth, TTenth Tenth, TEleventh Eleventh, TTwelfth Twelfth, TThirteenth Thirteenth)>>(Zip<TFirst, TSecond, TThird, TFourth, TFifth, TSixth, TSeventh, TEighth, TNinth, TTenth, TEleventh, TTwelfth, TThirteenth>).Method,
                    first.Expression,
                    GetSourceExpression(second),
                    GetSourceExpression(third),
                    GetSourceExpression(fourth),
                    GetSourceExpression(fifth),
                    GetSourceExpression(sixth),
                    GetSourceExpression(seventh),
                    GetSourceExpression(eighth),
                    GetSourceExpression(ninth),
                    GetSourceExpression(tenth),
                    GetSourceExpression(eleventh),
                    GetSourceExpression(twelfth),
                    GetSourceExpression(thirteenth)

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