facebook/react · error · Error

577

577

Error message

Invalid Iterator initializer.

What it means

extractIterator revives a serialized iterator by calling Symbol.iterator on the decoded model, which must be an array. A non-array model (corrupted, forged, or version-skewed payload) fails the isArray check and throws.

Source

Thrown at packages/react-server/src/ReactFlightReplyServer.js:1203

  return map;
}

function createSet(response: Response, model: Array<any>): Set<any> {
  if (!isArray(model)) {
    throw new Error('Invalid Set initializer.');
  }
  if ((model as any).$$consumed === true) {
    throw new Error('Already initialized Set.');
  }
  // This needs to come first to prevent the model from being consumed again in case of a cyclic reference.
  (model as any).$$consumed = true;
  const set = new Set(model);
  return set;
}

function extractIterator(response: Response, model: Array<any>): Iterator<any> {
  if (!isArray(model)) {
    throw new Error('Invalid Iterator initializer.');
  }
  if ((model as any).$$consumed === true) {
    throw new Error('Already initialized Iterator.');
  }
  // This needs to come first to prevent the model from being consumed again in case of a cyclic reference.
  (model as any).$$consumed = true;
  // $FlowFixMe[incompatible-use]: This uses raw Symbols because we're extracting from a native array.
  const iterator = model[Symbol.iterator]();
  return iterator;
}

function createModel(
  response: Response,
  model: any,
  parentObject: Object,
  key: string,
): any {
  if (key === 'then' && typeof model === 'function') {

View on GitHub (pinned to eafeac097b)

Solutions

1. Treat the decode failure as a bad request (catch and return 400).
2. Verify React version parity across the client/server boundary.
3. Rate-limit and log clients that repeatedly send malformed payloads.

try {
  const args = await decodeReply(formData);
} catch (e) {
  return new Response('Bad request', {status: 400});
}

When it happens

Trigger: The reply marks a chunk as an iterator but its backing model parses to a non-array value - truncated multipart bodies, hand-crafted submissions, or a different React version's wire format.

Common situations: Malformed requests hitting an action endpoint; middleware corrupting the payload; clients and servers built from different React releases.

Related errors

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• 574
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AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/ece45282bbfef434. Report an issue: GitHub.