mongodb/node-mongodb-native · critical · MongoRuntimeError

Unexpected null selectedServer. A cursor creating command…

Error message

Unexpected null selectedServer. A cursor creating command should have set this

What it means

Thrown as a MongoRuntimeError from getMore() when selectedServer is null/undefined at getMore time. selectedServer is populated during cursorInit() from the InitialCursorResponse returned by _initialize(); a null value here means the cursor creating command never stored the server it selected, which violates the documented contract of the @internal InitialCursorResponse interface. Like the other getMore() guards, this is a driver-internal invariant and indicates a bug rather than user misconfiguration.

Solutions

1. If subclassing, ensure _initialize() returns { server, session, response } with a non-null Server.
2. Report suspected driver bugs with a reproduction and the driver version.
3. Avoid calling the @internal getMore() directly; use the public iteration API.
4. Upgrade to the latest patch release.

try {
  for await (const doc of cursor) handle(doc);
} catch (err) {
  if (err instanceof MongoRuntimeError && /null selectedServer/.test(err.message)) {
    // recreate the cursor and retry the read
    cursor = collection.find(filter, opts);
  } else {
    throw err;
  }
}

When it happens

Trigger: A subclass's _initialize() returns an InitialCursorResponse without a server; the operation object failed to store its selected server (the TODO(NODE-2882) comment in the file describes this contract); test code constructs a response object manually without a server.

Common situations: Custom cursor subclasses that return incomplete InitialCursorResponse objects; driver bugs where server selection state is lost between operations; instrumentation that strips fields off the response.

Related errors

• Unexpected null cursor id. A cursor creating command should…
• Unexpected null session. A cursor creating command should…
• A collection name must be determined before getMore
• A collection name must be determined before killCursors
• Argument for maxTimeMS must be a number

AI-assisted analysis of mongodb/node-mongodb-native@dce7939f86 (2026-08-11). Data as JSON: /api/errors/9cab7e9773bf0ebd. Report an issue: GitHub.

Appendix: source

Thrown at src/cursor/abstract_cursor.ts:866

  /**
   * Returns a new uninitialized copy of this cursor, with options matching those that have been set on the current instance
   */
  abstract clone(): AbstractCursor<TSchema>;

  /** @internal */
  protected abstract _initialize(
    session: ClientSession | undefined
  ): Promise<InitialCursorResponse>;

  /** @internal */
  async getMore(): Promise<CursorResponse> {
    if (this.cursorId == null) {
      throw new MongoRuntimeError(
        'Unexpected null cursor id. A cursor creating command should have set this'
      );
    }
    if (this.selectedServer == null) {
      throw new MongoRuntimeError(
        'Unexpected null selectedServer. A cursor creating command should have set this'
      );
    }

    if (this.cursorSession == null) {
      throw new MongoRuntimeError(
        'Unexpected null session. A cursor creating command should have set this'
      );
    }
    const getMoreOptions = {
      ...this.cursorOptions,
      session: this.cursorSession,
      batchSize: this.cursorOptions.batchSize
    };

    const getMoreOperation = new GetMoreOperation(
      this.cursorNamespace,
      this.cursorId,

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