remotion-dev/remotion · error · Error

Expected size 4 for fact box, got ${size}

Error message

Expected size 4 for fact box, got ${size}

What it means

The WAV 'fact' chunk is spec-defined to hold exactly one uint32 (numberOfSamplesPerChannel), i.e. 4 payload bytes. parseFact reads the chunk size and rejects anything other than 4 because the parser only models the canonical fact payload. A non-4 size indicates a malformed, extended, or corrupted fact chunk.

Source

Thrown at packages/media-parser/src/containers/wav/parse-fact.ts:13

import type {ParseResult} from '../../parse-result';
import type {ParserState} from '../../state/parser-state';
import type {WavFact} from './types';

export const parseFact = ({
	state,
}: {
	state: ParserState;
}): Promise<ParseResult> => {
	const {iterator} = state;
	const size = iterator.getUint32Le();
	if (size !== 4) {
		throw new Error(`Expected size 4 for fact box, got ${size}`);
	}

	const numberOfSamplesPerChannel = iterator.getUint32Le();
	const factBox: WavFact = {
		type: 'wav-fact',
		numberOfSamplesPerChannel,
	};
	state.structure.getWavStructure().boxes.push(factBox);
	return Promise.resolve(null);
};

View on GitHub (pinned to 78fe4bb3fd)

Solutions

1. Inspect the fact chunk size in a hex editor or via ffprobe -show_format.
2. Re-encode to a clean WAV: ffmpeg -i in.wav out.wav.
3. Use Mediabunny for broader WAV compatibility.
4. try/catch parseMedia() and treat the file as unsupported.

Example fix

# re-encode to a spec-clean WAV
ffmpeg -i in.wav -c:a pcm_s16le out.wav

// then handle failure gracefully
try { await parseMedia({ src: 'out.wav' }); } catch { /* unsupported */ }

// ffprobe check: a spec-clean fact chunk parses without warning
// ffprobe -v warning in.wav   (exit 0 + no warnings => fact chunk ok)

try {
  await parseMedia({ src: 'in.wav' });
} catch (err) {
  if (err instanceof Error && /fact box/i.test(err.message)) {
    // non-standard fact chunk — re-encode or skip
  } else throw err;
}

When it happens

Trigger: parseMedia() on a WAV whose 'fact' chunk size field is not 4: a padded/extended fact chunk, a corrupted size word, or an encoder that emits a non-standard fact body.

Common situations: Non-compliant encoders; broadcast/DAW WAVs with extended fact data; hand-edited or partially-overwritten files; files passed through lossy conversion.

Related errors

• Expected data box
• Only supporting WAVE with 22 extra bytes, but got ${extraSiz
• Only supporting WAVE with PCM audio format, but got ${subFor
• Expected fmt box
• Unexpected sampling frequency index ${samplingFrequencyIndex

AI-assisted analysis of remotion-dev/remotion@78fe4bb3fd (2026-08-12). Data as JSON: /api/errors/8414a86fcd5b4ca4. Report an issue: GitHub.