Cysharp/UniTask · error · ArgumentNullException

observer

Error message

observer

What it means

Thrown by AsyncSubject<T>.Subscribe(IObserver<T>) (line 457) when the observer argument is null. Reactive Extensions require a non-null observer for every subscription, so UniTask enforces it with ArgumentNullException("observer"). This Subscribe is what runs when a caller subscribes to the observable produced by ToObservable().

Source

Thrown at src/UniTask/Assets/Plugins/UniTask/Runtime/UniTaskObservableExtensions.cs:457

            old.OnError(error);
        }

        public void OnNext(T value)
        {
            lock (observerLock)
            {
                ThrowIfDisposed();
                if (isStopped) return;

                this.hasValue = true;
                this.lastValue = value;
            }
        }

        public IDisposable Subscribe(IObserver<T> observer)
        {
            if (observer == null) throw new ArgumentNullException("observer");

            var ex = default(Exception);
            var v = default(T);
            var hv = false;

            lock (observerLock)
            {
                ThrowIfDisposed();
                if (!isStopped)
                {
                    var listObserver = outObserver as ListObserver<T>;
                    if (listObserver != null)
                    {
                        outObserver = listObserver.Add(observer);
                    }
                    else
                    {
                        var current = outObserver;

View on GitHub (pinned to ceac8d6946)

Solutions

1. Pass a non-null IObserver<T> (or valid OnNext/OnError handler lambdas).
2. If you only want the value, use observable.ToUniTask() instead of a raw observer.
3. Null-check the observer before calling Subscribe.

Example fix

// before
observable.Subscribe(null); // throws ArgumentNullException

// after
observable.Subscribe(x => Handle(x), ex => Log(ex));
// or simply:
var value = await observable.ToUniTask();

if (observer == null) throw new ArgumentNullException(nameof(observer));
observable.Subscribe(observer);

When it happens

Trigger: Calling .Subscribe(null) on the observable returned by ToObservable(), or passing an observer variable/expression that evaluated to null (a failed factory, a null-coalescing that returned null, or an accidental null lambda).

Common situations: Passing null in place of an observer; relying on an optional OnNext argument that defaulted to null; a Subscribe overload where the wrong slot was left null.

Related errors

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

AI-assisted analysis of Cysharp/UniTask@ceac8d6946 (2026-08-13). Data as JSON: /api/errors/358c095a9e8a277a. Report an issue: GitHub.