What causes "Value cannot be null. (Parameter 'eleventh')" in dotnet/reactive?

Check which argument position is null at the call site (Parameter 'eleventh' = the 11th argument) and fix the expression producing it.

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'eleventh')

Error message

Value cannot be null. (Parameter 'eleventh')

What it means

The 11-source CombineLatest overload for IQbservable sequences (System.Reactive/Linq/QbservableEx.Homoicon.cs:496) validates every source argument up front and throws ArgumentNullException when the eleventh IObservable<TEleventh> is null. Qbservable expression trees must be built from non-null source expressions because the provider serializes the whole call into an Expression.Call, so null sources are rejected eagerly rather than failing later during subscription.

Solutions

Check which argument position is null at the call site (Parameter 'eleventh' = the 11th argument) and fix the expression producing it.
Replace the null with a non-null sequence such as Observable.Empty<TEleventh>() or a default source from your IQbservableProvider.
Wrap the source-producing logic so it throws early with a meaningful message instead of returning null.
Add a Debug.Assert / guard in your own helper that builds the CombineLatest call.

Example fix

// before
var result = q1.CombineLatest(q2, ..., q10, GetSource()); // GetSource() returned null
// after
var src = GetSource() ?? QueryableEx.Empty<TEleventh>(provider);
var result = q1.CombineLatest(q2, ..., q10, src);

Defensive patterns

Strategy: validation

Validate before calling

if (eleventh == null) throw new InvalidOperationException("eleventh source must be a non-null IObservable<TEleventh>");
var result = q1.CombineLatest(q2, q3, q4, q5, q6, q7, q8, q9, q10, q11, eleventh);

Type guard

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

Try / catch

try { var merged = first.CombineLatest(..., eleventh); }
catch (ArgumentNullException ex) when (ex.ParamName == "eleventh") { log.Error("11th CombineLatest source is null"); throw; }

Prevention

Never return null from methods that produce observables - return Observable.Empty instead.
Validate all source arguments in your own composition helpers before delegating to Rx operators.
Keep CombineLatest argument counts small; prefer combining collections or nested calls.
Enable nullable reference types (<Nullable>enable</Nullable>) so null sources are caught at compile time.

When it happens

Trigger: Calling the public static CombineLatest<TFirst..TEleventh>(this IQbservable<TFirst> first, IObservable<TSecond> second, ..., IObservable<TEleventh> eleventh) extension with a null eleventh argument, e.g. when the last sequence is the result of a lookup or a conditional expression that evaluated to null.

Common situations: A helper method returns null instead of Observable.Empty or a Qbservable provider's default; a dictionary/config-driven source lookup misses a key; chaining results where an earlier query returned null instead of throwing.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/bc60f4cf901ec976. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:496

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

            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)>(
                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>, IQbservable<(TFirst First, TSecond Second, TThird Third, TFourth Fourth, TFifth Fifth, TSixth Sixth, TSeventh Seventh, TEighth Eighth, TNinth Ninth, TTenth Tenth, TEleventh Eleventh)>>(CombineLatest<TFirst, TSecond, TThird, TFourth, TFifth, TSixth, TSeventh, TEighth, TNinth, TTenth, TEleventh>).Method,
                    first.Expression,
                    GetSourceExpression(second),
                    GetSourceExpression(third),
                    GetSourceExpression(fourth),
                    GetSourceExpression(fifth),
                    GetSourceExpression(sixth),
                    GetSourceExpression(seventh),
                    GetSourceExpression(eighth),
                    GetSourceExpression(ninth),
                    GetSourceExpression(tenth),
                    GetSourceExpression(eleventh)
                )
            );

View on GitHub (pinned to 94b5d5ab91)