sveltejs/kit · error · Error

Invalid data for Map reviver

Error message

Invalid data for Map reviver

What it means

When deserializing remote function arguments, a value tagged as a Map must be an array (of [key, value] string pairs). Anything else — an object, string, or non-array — indicates corrupted or forged payload data, so the reviver throws.

Source

Thrown at packages/kit/src/runtime/shared.js:182

		};
	}

	const all_reducers = { ...encoders, ...remote_fns_reducers };

	/** @type {(value: unknown) => string} */
	const stringify = (value) => devalue.stringify(value, all_reducers);

	return all_reducers;
}

function create_remote_arg_revivers() {
	const remote_fns_revivers = {
		/** @type {(value: unknown) => unknown} */
		[remote_object]: (value) => value,
		/** @type {(value: unknown) => Map<unknown, unknown>} */
		[remote_map]: (value) => {
			if (!Array.isArray(value)) {
				throw new Error('Invalid data for Map reviver');
			}

			const map = new Map();

			for (const item of value) {
				if (
					!Array.isArray(item) ||
					item.length !== 2 ||
					typeof item[0] !== 'string' ||
					typeof item[1] !== 'string'
				) {
					throw new Error('Invalid data for Map reviver');
				}
				const [key, val] = item;
				map.set(parse(key), parse(val));
			}

			return map;

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Send Map arguments through the standard remote-function client (don't hand-build request bodies)
2. Update @sveltejs/kit on client and server so serialization versions match
3. If testing, mock at the function level rather than reshaping the wire format

function isValidMapPayload(v) { return Array.isArray(v); }

function isMapWireFormat(v) {
  return Array.isArray(v) && v.every((e) => Array.isArray(e) && e.length === 2 && typeof e[0] === 'string' && typeof e[1] === 'string');
}

try {
  const result = await remoteFn(args);
} catch (e) {
  if (e.message.includes('Map reviver')) console.error('Malformed Map argument payload — resend via official client');
}

When it happens

Trigger: A request payload where the remote_map-tagged value is not an array; hand-crafted or middleware-mutated request bodies; version mismatch between client serializer and server reviver.

Common situations: Proxies or test harnesses re-serializing request JSON incorrectly; custom fetch mocks returning reshaped arguments to remote functions.

Related errors

• Multiple values provided for ${name} header where only one e
• The ${protocol_header} header specified ${protocol} which is
• The ${port_header} header specified ${port} which is an inva
• The `csp.directives['trusted-types']` option must include 's
• Could not get the request store.

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/7b7a4bcc087e80b1. Report an issue: GitHub.