remotion-dev/remotion · error · Error

Expected block segment

Error message

Expected block segment

What it means

While post-processing a BlockGroup, the parser looks for a SimpleBlock or Block child and throws if none (or a non-block child matched) is found. Per the Matroska spec a BlockGroup must contain a Block, so this indicates a malformed BlockGroup element.

Source

Thrown at packages/media-parser/src/containers/webm/parse-ebml.ts:278

			return {
				type: 'Block',
				value: new Uint8Array([]),
				minVintWidth: ebml.minVintWidth,
			};
		}
	}

	if (ebml.type === 'BlockGroup') {
		// Blocks don't have information about keyframes.
		// https://ffmpeg.org/pipermail/ffmpeg-devel/2015-June/173825.html
		// "For Blocks, keyframes is
		// inferred by the absence of ReferenceBlock element (as done by matroskadec).""

		const block = ebml.value.find(
			(c) => c.type === 'SimpleBlock' || c.type === 'Block',
		);
		if (!block || (block.type !== 'SimpleBlock' && block.type !== 'Block')) {
			throw new Error('Expected block segment');
		}

		const hasReferenceBlock = ebml.value.find(
			(c) => c.type === 'ReferenceBlock',
		);

		const sample =
			block.value.length === 0
				? null
				: await getSampleFromBlock({
						ebml: block,
						webmState,
						offset,
						structureState,
						callbacks,
						logLevel,
						onVideoTrack,
						avcState,

View on GitHub (pinned to 78fe4bb3fd)

Solutions

1. Re-mux the file ('ffmpeg -i in.mkv -c copy out.mkv') to reconstruct valid BlockGroups.
2. Validate with ffprobe; if it errors on the same region, the file is damaged.
3. Use a different source file.
4. Try Mediabunny as an alternate parser.

try {
  await parseMedia({src: 'in.mkv', fields: {samples: true}});
} catch (err) {
  if (err instanceof Error && /Expected block segment/.test(err.message)) {
    throw new Error('Malformed BlockGroup in source; re-mux with ffmpeg.', {cause: err});
  }
  throw err;
}

When it happens

Trigger: A BlockGroup element whose children are only ReferenceBlock/BlockAdditions/etc. with no actual Block/SimpleBlock; a partial BlockGroup cut by truncation; corruption that renamed the child element ID.

Common situations: Damaged MKV files, files produced by non-conformant muxers, or streams truncated inside a BlockGroup.

Related errors

• Could not find sample rate or number of channels
• Expected track entry segment
• Not enough bytes left to parse EBML - this should not happen
• Offset ${offsetNow - startOffset} is larger than the length
• Expected segment

AI-assisted analysis of remotion-dev/remotion@78fe4bb3fd (2026-08-12). Data as JSON: /api/errors/cacb2b96cda31bb5. Report an issue: GitHub.