dotnet/aspnetcore · error · Error

Invalid payload for StreamItem message.

Error message

Invalid payload for StreamItem message.

What it means

Thrown by JsonHubProtocol._isStreamItemMessage when a StreamItem message has an undefined `item` field. The validator already checks invocationId via _assertNotEmptyString; reaching the second throw means the invocationId passed but the payload carries no streamed item, which is meaningless for a StreamItem.

Solutions

1. On the server, ensure streaming methods yield concrete values (or a sentinel) rather than undefined.
2. Configure the serializer to include null fields if item may legitimately be null.
3. Verify the hub method return type is IEnumerable/IAsyncEnumerable<T> with a non-null T.

function validStreamItem(m: any): boolean {
  return typeof m.invocationId === "string" && m.invocationId !== "" && m.item !== undefined;
}

function isCompleteStreamItem(m: any): boolean {
  return typeof m?.invocationId === "string" && m.invocationId.length > 0 && m.item !== undefined;
}

When it happens

Trigger: Server (or a malformed/injected frame) sends a JSON message with `type: 2` (StreamItem), a valid invocationId, but `item` is omitted/null. The client refuses to surface an empty stream item.

Common situations: Server-side hub method yields undefined for a streaming result, a serializer strips null-valued properties, or a custom server implementation that does not populate `item`.

Related errors

• Error reading JSON.
• errorMessage
• Invalid input for JSON hub protocol. Expected a string.
• Invalid payload.
• Invalid payload for Completion message.

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/cfe74a125a577e10. Report an issue: GitHub.

Appendix: source

Thrown at src/SignalR/clients/ts/signalr/src/JsonHubProtocol.ts:105

     * @returns {string} A string containing the serialized representation of the message.
     */
    public writeMessage(message: HubMessage): string {
        return TextMessageFormat.write(JSON.stringify(message));
    }

    private _isInvocationMessage(message: InvocationMessage): void {
        this._assertNotEmptyString(message.target, "Invalid payload for Invocation message.");

        if (message.invocationId !== undefined) {
            this._assertNotEmptyString(message.invocationId, "Invalid payload for Invocation message.");
        }
    }

    private _isStreamItemMessage(message: StreamItemMessage): void {
        this._assertNotEmptyString(message.invocationId, "Invalid payload for StreamItem message.");

        if (message.item === undefined) {
            throw new Error("Invalid payload for StreamItem message.");
        }
    }

    private _isCompletionMessage(message: CompletionMessage): void {
        if (message.result && message.error) {
            throw new Error("Invalid payload for Completion message.");
        }

        if (!message.result && message.error) {
            this._assertNotEmptyString(message.error, "Invalid payload for Completion message.");
        }

        this._assertNotEmptyString(message.invocationId, "Invalid payload for Completion message.");
    }

    private _isAckMessage(message: AckMessage): void {
        if (typeof message.sequenceId !== 'number') {
            throw new Error("Invalid SequenceId for Ack message.");

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