What causes "Value cannot be null. (Parameter 'source14')" in dotnet/reactive?

Assign a real IObservable to the 14th argument before calling Zip.

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source14')

Error message

Value cannot be null. (Parameter 'source14')

What it means

ArgumentNullException (Parameter 'source14'): the 14th observable argument to the 16-ary Observable.Zip overload is null. The implementation checks every source sequentially and throws with the failing parameter name, so this error pinpoints exactly which slot was null.

Solutions

Assign a real IObservable<T> to the 14th argument before calling Zip.
Replace the null with Observable.Empty<T>()/Observable.Never<T>() to encode 'absent'.
Add a debug-time assertion enumerating all sources for nulls.
Refactor to the params/IEnumerable Zip overload for variable arity.

Example fix

// before
Zip(..., s13, s14 /* null */, s15, ..., sel);
// after
Zip(..., s13, s14 ?? Observable.Empty<decimal>(), s15, ..., sel);

Defensive patterns

Strategy: validation

Validate before calling

if (source14 == null) throw new ArgumentNullException(nameof(source14));
// or:
var s14 = source14 ?? Observable.Never<T14>();

Type guard

static IObservable<T> NonNull<T>(IObservable<T>? o) => o ?? Observable.Empty<T>();
var s14 = NonNull(source14);

Try / catch

try
{
    var zipped = Observable.Zip(source1, /* ... */ source14, /* ... */ source16, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == nameof(source14))
{
    logger.LogError(ex, "source14 was null when composing Zip");
}

Prevention

Use factories that guarantee non-null observables for all feature-flag states.
When growing a zip's arity, update all source variables in the same change.
Add a debug assertion scanning all zip inputs for nulls in test builds.
Prefer building sources into a list and using the collection Zip overload.

When it happens

Trigger: Observable.Zip(source1..source16, resultSelector) called with source14 == null, e.g. a nullable backing field, a dictionary TryGetValue path that ignored the miss, or a conditional build of sources leaving a gap.

Common situations: Caching layers returning null on cold start, feature flags removing a stream but leaving the zip arity unchanged, or copy-paste of a 14-source zip where the new source variable was never assigned.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/628ce87ebf726cbe. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:1159

            if (source11 == null)
            {
                throw new ArgumentNullException(nameof(source11));
            }

            if (source12 == null)
            {
                throw new ArgumentNullException(nameof(source12));
            }

            if (source13 == null)
            {
                throw new ArgumentNullException(nameof(source13));
            }

            if (source14 == null)
            {
                throw new ArgumentNullException(nameof(source14));
            }

            if (source15 == null)
            {
                throw new ArgumentNullException(nameof(source15));
            }

            if (source16 == null)
            {
                throw new ArgumentNullException(nameof(source16));
            }

            if (resultSelector == null)
            {
                throw new ArgumentNullException(nameof(resultSelector));
            }

            return s_impl.Zip(source1, source2, source3, source4, source5, source6, source7, source8, source9, source10, source11, source12, source13, source14, source15, source16, resultSelector);

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