Cysharp/UniTask · error · ArgumentNullException

error

Error message

error

What it means

Thrown by AsyncSubject<T>.OnError(Exception) (line 426) when the supplied exception is null. This enforces the Reactive Extensions contract that OnError must never receive null — an error sequence must carry an actual error — using ArgumentNullException("error"). The subject is the one returned from UniTask<T>.ToObservable().

Source

Thrown at src/UniTask/Assets/Plugins/UniTask/Runtime/UniTaskObservableExtensions.cs:426

                isStopped = true;
                v = lastValue;
                hv = hasValue;
            }

            if (hv)
            {
                old.OnNext(v);
                old.OnCompleted();
            }
            else
            {
                old.OnCompleted();
            }
        }

        public void OnError(Exception error)
        {
            if (error == null) throw new ArgumentNullException("error");

            IObserver<T> old;
            lock (observerLock)
            {
                ThrowIfDisposed();
                if (isStopped) return;

                old = outObserver;
                outObserver = EmptyObserver<T>.Instance;
                isStopped = true;
                lastError = error;
            }

            old.OnError(error);
        }

        public void OnNext(T value)
        {

View on GitHub (pinned to ceac8d6946)

Solutions

1. Pass a real Exception instance to OnError; never null.
2. If you have no concrete error, complete the sequence with OnCompleted() instead of OnError.
3. Null-check the exception at the boundary before forwarding it into OnError.

Example fix

// before
subject.OnError(GetErrorOrNull()); // null -> throws ArgumentNullException

// after
var ex = GetErrorOrNull();
if (ex != null) subject.OnError(ex);
else subject.OnCompleted();

if (error == null) throw new ArgumentException("error must not be null", nameof(error));
subject.OnError(error);

When it happens

Trigger: Calling OnError(null) on the subject returned from ToObservable(), or wiring an upstream Rx source/observer that incorrectly propagates a null exception into OnError.

Common situations: Manually driving an AsyncSubject with an exception that resolved to null (e.g. a factory returning null on success); a broken custom IObservable that calls observer.OnError(null); forwarding an unchecked exception variable.

Related errors

• observer
• AsyncSubject is not completed yet
• Disposable is already set
• Can not trigger itself in iterating.
• handler

AI-assisted analysis of Cysharp/UniTask@ceac8d6946 (2026-08-13). Data as JSON: /api/errors/e34fb56677fdfb8b. Report an issue: GitHub.