mermaid-js/mermaid · error · Error

Block start ${block.start} is greater than block end ${block

Error message

Block start ${block.start} is greater than block end ${block.end}.

What it means

Thrown by getNextFittingBlock after the undefined checks pass but block.start is greater than block.end. Since the populate loop already rejects user-supplied start>end and auto-computes end = start+bits-1 (which is >= start for bits>=1), reaching this guard means an internal logic error produced an inverted range — most likely from the multi-row split arithmetic in getNextFittingBlock itself.

Source

Thrown at packages/mermaid/src/diagrams/packet/parser.ts:68

    }
  }
  db.pushWord(word);
};

const getNextFittingBlock = (
  block: PacketBlock,
  row: number,
  bitsPerRow: number
): [Required<PacketBlock>, PacketBlock | undefined] => {
  if (block.start === undefined) {
    throw new Error('start should have been set during first phase');
  }
  if (block.end === undefined) {
    throw new Error('end should have been set during first phase');
  }

  if (block.start > block.end) {
    throw new Error(`Block start ${block.start} is greater than block end ${block.end}.`);
  }

  if (block.end + 1 <= row * bitsPerRow) {
    return [block as Required<PacketBlock>, undefined];
  }

  const rowEnd = row * bitsPerRow - 1;
  const rowStart = row * bitsPerRow;
  return [
    {
      start: block.start,
      end: rowEnd,
      label: block.label,
      bits: rowEnd - block.start,
    },
    {
      start: rowStart,
      end: block.end,

View on GitHub (pinned to d93e9c88c0)

Solutions

1. Report upstream with the exact packet-beta input that reproduces and the mermaid version.
2. Upgrade mermaid.
3. If reproducing in a fork, log block.start/block.end at the split boundary to find the arithmetic error.
4. Simplify the packet layout to single-row blocks as a workaround while the bug is fixed.

function blockStartLeEnd(block: {start:number;end:number}): boolean {
  return block.start <= block.end;
}

try {
  await mermaid.parse(text);
} catch (e) {
  if (e instanceof Error && /Block start .* is greater than block end/.test(e.message)) {
    reportBug(e, text);
  } else throw e;
}

When it happens

Trigger: A block spanning multiple rows where the split computation yields start>end; an internal regression in the rowEnd/rowStart math; programmatic block construction with start>end that bypassed the populate guards.

Common situations: Not triggerable from valid packet-beta text in stable releases; appears with edge-case block sizes near row boundaries or after internal refactors.

Related errors

• start should have been set during first phase
• end should have been set during first phase
• parser.parser?.yy was not a PacketDB. This is due to a bug w
• Packet block ${start} - ${end} is invalid. End must be great
• Packet block ${start} - ${end ?? start} is not contiguous. I

AI-assisted analysis of mermaid-js/mermaid@d93e9c88c0 (2026-08-12). Data as JSON: /api/errors/d0e445d7cde469b8. Report an issue: GitHub.