dotnet/aspnetcore · error · Error

Expected data to be of type

Error message

Expected data to be of type ${typeof(this._buffer)} but was of type ${typeof(data)}

What it means

Thrown by the send-loop's _bufferData helper when a new chunk's type does not match the type of data already buffered. The buffer is homogeneous — once you push a string you must keep pushing strings, and once you push an ArrayBuffer you must keep pushing ArrayBuffers. Mixing types breaks the framing contract with the underlying transport.

Solutions

1. Pick one hub protocol per connection and stick to its transfer format (Text -> strings, Binary -> ArrayBuffer).
2. Encode strings to ArrayBuffer (TextEncoder) only when using a binary protocol, and vice-versa.
3. Do not reuse a connection across protocol changes; rebuild it with the desired protocol.

Example fix

// before - mixing types on one connection
conn.send("Method", "text");
conn.send("Method", new Uint8Array([1,2,3]));
// after - keep the buffer homogeneous for the chosen protocol
conn.send("Method", "text");
conn.send("Method", "more-text");

function ensureHomogeneous(buffer: unknown[], next: string | ArrayBuffer): void {
  if (buffer.length && typeof buffer[0] !== typeof next) {
    throw new TypeError("Refusing to mix string and ArrayBuffer payloads");
  }
}

function isStringPayload(p: unknown): p is string {
  return typeof p === "string";
}

When it happens

Trigger: Calling `connection.send(...)` / `hubConnection.send` (or stream ingestion) with payloads whose JS type alternates between `string` and `ArrayBuffer`/`Uint8Array` on the same connection instance within the buffered send window.

Common situations: Using a JSON hub protocol (text) but occasionally sending a raw byte payload, or switching hub protocols (JsonHubProtocol <-> MessagePackHubProtocol) on an already-established connection, or sending a Uint8Array where the buffer expected a string.

Related errors

• Binary protocols over XmlHttpRequest not implementing…
• Message is incomplete.
• Message is incomplete.
• Unknown value: .
• Unknown transport: .

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/46caf817c6f2757a. Report an issue: GitHub.

Appendix: source

Thrown at src/SignalR/clients/ts/signalr/src/HttpConnection.ts:762

    }

    public send(data: string | ArrayBuffer): Promise<void> {
        this._bufferData(data);
        if (!this._transportResult) {
            this._transportResult = new PromiseSource();
        }
        return this._transportResult.promise;
    }

    public stop(): Promise<void> {
        this._executing = false;
        this._sendBufferedData.resolve();
        return this._sendLoopPromise;
    }

    private _bufferData(data: string | ArrayBuffer): void {
        if (this._buffer.length && typeof(this._buffer[0]) !== typeof(data)) {
            throw new Error(`Expected data to be of type ${typeof(this._buffer)} but was of type ${typeof(data)}`);
        }

        this._buffer.push(data);
        this._sendBufferedData.resolve();
    }

    private async _sendLoop(): Promise<void> {
        while (true) {
            await this._sendBufferedData.promise;

            if (!this._executing) {
                if (this._transportResult) {
                    this._transportResult.reject("Connection stopped.");
                }

                break;
            }

View on GitHub (pinned to 3600ca084e)