dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'twelfth')

Error message

Value cannot be null. (Parameter 'twelfth')

What it means

This ArgumentNullException is thrown by the 12-source Qbservable.CombineLatest overload when the twelfth (last) source sequence is null. Like all Qbservable operators, CombineLatest eagerly null-checks every source before constructing the Expression.Call that describes the query, so a null twelfth argument fails immediately at call time. The parameter name in the exception points at the offending argument.

Solutions

1. Ensure the twelfth argument is a non-null IObservable<TTwelfth> before the call.
2. Coalesce to a default sequence such as Observable.Empty<TTwelfth>() when the source is optional.
3. Fix the producer of the twelfth sequence so it never returns null.

Example fix

// before
var query = first.CombineLatest(s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12);

// after
var query = first.CombineLatest(s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12 ?? Observable.Empty<TTwelfth>());

if (s12 == null) throw new InvalidOperationException("twelfth CombineLatest source is null"); // or s12 = s12 ?? Observable.Empty<TTwelfth>();

bool IsValidSource<T>(IObservable<T> s) => s is not null;

try { var query = first.CombineLatest(..., s12); } catch (ArgumentNullException ex) when (ex.ParamName == "twelfth") { /* substitute default source */ }

When it happens

Trigger: Calling the 12-argument CombineLatest extension (Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:572) with null for the twelfth IObservable<TTwelfth> argument.

Common situations: The last source in a long CombineLatest chain is frequently the one forgotten after a signature change or when it comes from an optional service/config that resolved to null.

Related errors

• Value cannot be null. (Parameter 'eleventh')
• ArgumentNullException(nameof(second))
• ArgumentNullException(nameof(sources))
• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/98717586c7bd8822. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:572

                throw new ArgumentNullException(nameof(third));
            if (fourth == null)
                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));

            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)>(
                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>, 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)>>(CombineLatest<TFirst, TSecond, TThird, TFourth, TFifth, TSixth, TSeventh, TEighth, TNinth, TTenth, TEleventh, TTwelfth>).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)
                )

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