mozilla/pdf.js · error · FormatError

Malformed CMap: expected string.

Error message

Malformed CMap: expected string.

What it means

Thrown by the expectString helper while parsing a CMap when the lexer yields an object that is not a JS string at a position where a hex/string token is mandatory (for example the source code points inside a beginbfchar block). It signals a structurally malformed CMap stream rather than a recoverable typo.

Source

Thrown at src/core/cmap.js:452

  }

  // eslint-disable-next-line getter-return
  get isIdentityCMap() {
    unreachable("should not access .isIdentityCMap");
  }
}

function strToInt(str) {
  let a = 0;
  for (let i = 0; i < str.length; i++) {
    a = (a << 8) | str.charCodeAt(i);
  }
  return a >>> 0;
}

function expectString(obj) {
  if (typeof obj !== "string") {
    throw new FormatError("Malformed CMap: expected string.");
  }
}

function expectInt(obj) {
  if (!Number.isInteger(obj)) {
    throw new FormatError("Malformed CMap: expected int.");
  }
}

function parseBfChar(cMap, lexer) {
  while (true) {
    let obj = lexer.getObj();
    if (obj === EOF) {
      break;
    }
    if (isCmd(obj, "endbfchar")) {
      return;
    }

View on GitHub (pinned to 5903d58d58)

Solutions

1. Validate the PDF in an authoritative viewer and regenerate it if it fails.
2. Update pdf.js.
3. Catch FormatError around document load and skip ToUnicode for the offending font.

try {
  await loadingTask.promise;
} catch (e) {
  if (e instanceof FormatError && /expected string/.test(e.message)) {
    console.warn('Malformed CMap (expected string), text selection may degrade', e);
  } else throw e;
}

When it happens

Trigger: Parsing a CMap stream where a token expected to be a hex string such as <0041> is instead a number, a Name, a Cmd, or EOF. Reached via parseBfChar / parseCidChar / similar parse* helpers that call expectString.

Common situations: Truncated ToUnicode stream; a PDF whose /ToUnicode entry points at a non-CMap stream; a broken CMap writer that emitted numbers where strings were required.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• mapBfRange - ignoring data above MAX_MAP_RANGE.
• mapBfRangeToArray - ignoring data above MAX_MAP_RANGE.
• Invalid bf range.
• mapCidRange - ignoring data above MAX_MAP_RANGE.
• Malformed CMap: expected int.

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/db46023298cd0146. Report an issue: GitHub.