mermaid-js/mermaid · error · Error

Packet block is invalid. Cannot have a zero bit field.

Error message

Packet block ${start} is invalid. Cannot have a zero bit field.

What it means

Thrown by the packet populate loop when a block's bits field is exactly 0. A zero-width field has no bits to render and is invalid; the check runs after start/end contiguity is verified, so it specifically catches an explicit bits: 0 (or a computed zero via end-start+1 when end<start after the earlier guard, though that path is already rejected).

Solutions

1. Set bits to at least 1, or remove the block entirely if it represents nothing.
2. If you need a visual spacer, use a small non-zero reserved block instead.
3. Check any code generator that emits packet syntax and ensure it never produces bits: 0.

Example fix

// before
packet-beta
  0-7: Field A
  8-7: Zero width   // bits computes to 0

// after
packet-beta
  0-7: Field A
  8: Single bit field

function validateNoZeroBits(blocks: {start?:number;end?:number;bits?:number}[]): string[] {
  const errors: string[] = [];
  for (const b of blocks) {
    if (b.bits === 0) errors.push(`Block at ${b.start ?? '?'} has zero bits`);
  }
  return errors;
}

function hasPositiveBits(bits: number | undefined): boolean {
  return bits === undefined || bits > 0;
}

try {
  await mermaid.render('g', diagramText);
} catch (e) {
  if (e instanceof Error && /Cannot have a zero bit field/.test(e.message)) {
    showUserError('A packet block declares zero bits. Set bits >= 1 or remove the block.');
  } else {
    throw e;
  }
}

When it happens

Trigger: Declaring a packet block with bits: 0; writing a range where the computed width collapses to zero.

Common situations: Placeholder/reserved field left as zero by mistake; templating system emitting bits:0 for optional fields; misunderstanding that every block must consume at least one bit.

Related errors

• Packet block - is invalid. End must be greater than start.
• Packet block - is not contiguous. It should start from .
• Block start is greater than block end .
• end should have been set during first phase
• " " has invalid value: . Negative values are not allowed in…

AI-assisted analysis of mermaid-js/mermaid@d93e9c88c0 (2026-08-12). Data as JSON: /api/errors/90c6f3b7fc545a30. Report an issue: GitHub.

Appendix: source

Thrown at packages/mermaid/src/diagrams/packet/parser.ts:31

  let lastBit = -1;
  let word: PacketWord = [];
  let row = 1;
  const { bitsPerRow } = db.getConfig();

  for (let { start, end, bits, label } of ast.blocks) {
    if (start !== undefined && end !== undefined && end < start) {
      throw new Error(`Packet block ${start} - ${end} is invalid. End must be greater than start.`);
    }
    start ??= lastBit + 1;
    if (start !== lastBit + 1) {
      throw new Error(
        `Packet block ${start} - ${end ?? start} is not contiguous. It should start from ${
          lastBit + 1
        }.`
      );
    }
    if (bits === 0) {
      throw new Error(`Packet block ${start} is invalid. Cannot have a zero bit field.`);
    }
    end ??= start + (bits ?? 1) - 1;
    bits ??= end - start + 1;
    lastBit = end;
    log.debug(`Packet block ${start} - ${lastBit} with label ${label}`);

    while (word.length <= bitsPerRow + 1 && db.getPacket().length < maxPacketSize) {
      const [block, nextBlock] = getNextFittingBlock({ start, end, bits, label }, row, bitsPerRow);
      word.push(block);
      if (block.end + 1 === row * bitsPerRow) {
        db.pushWord(word);
        word = [];
        row++;
      }
      if (!nextBlock) {
        break;
      }
      ({ start, end, bits, label } = nextBlock);

View on GitHub (pinned to d93e9c88c0)