mozilla/pdf.js · error · FormatError

Base font is not specified

Error message

Base font is not specified

What it means

Thrown by translateFont() when a non-composite, non-Type3 font has no /FontDescriptor AND its /BaseFont entry is not a Name. This is a legacy-compatibility path (pre-PDF-1.5 base-14 fonts like Helvetica did not require a FontDescriptor), where PDF.js falls back to synthesizing a descriptor from the BaseFont name; if even that name is missing or not a Name, the font is unrecoverable. The guard precedes normalizeFontName() which would otherwise crash on a non-string.

Source

Thrown at src/core/evaluator.js:4482

    if (!descriptor) {
      if (isType3Font) {
        // FontDescriptor is only required for Type3 fonts when the document
        // is a tagged pdf.
        descriptor = Dict.empty;
      } else if (composite) {
        // Some PDFs omit the FontDescriptor on the descendant CIDFont when
        // referencing one of the standard Acrobat CJK fonts via a predefined
        // CMap (e.g. /Encoding /90ms-RKSJ-H with /BaseFont /HeiseiMin-W3).
        // Fall through so the CMap is loaded by the composite-font path
        // below; otherwise multi-byte codes would be decoded byte-by-byte.
        descriptor = Dict.empty;
      } else {
        // Before PDF 1.5 if the font was one of the base 14 fonts, having a
        // FontDescriptor was not required.
        // This case is here for compatibility.
        let baseFontName = dict.get("BaseFont");
        if (!(baseFontName instanceof Name)) {
          throw new FormatError("Base font is not specified");
        }

        // Using base font name as a font name.
        baseFontName = normalizeFontName(baseFontName.name);
        const metrics = this.getBaseFontMetrics(baseFontName);

        // Simulating descriptor flags attribute
        const fontNameWoStyle = baseFontName.split("-", 1)[0];
        const flags =
          (this.isSerifFont(fontNameWoStyle) ? FontFlags.Serif : 0) |
          (metrics.monospace ? FontFlags.FixedPitch : 0) |
          (getSymbolsFonts()[fontNameWoStyle]
            ? FontFlags.Symbolic
            : FontFlags.Nonsymbolic);

        const properties = {
          type,
          name: baseFontName,

View on GitHub (pinned to 5903d58d58)

Solutions

1. Re-export the PDF from the source application or through Ghostscript to populate /BaseFont and /FontDescriptor.
2. If authoring, always include /BaseFont as a Name on every font dictionary.
3. Isolate the failing page with stopAtErrors: true and continue rendering others.
4. If the document opens in Acrobat, file a PDF.js bug — additional fallback heuristics may be possible.

try {
  await page.render({ canvasContext, viewport });
} catch (e) {
  if (e.name === 'FormatError' && /Base font is not specified/.test(e.message)) {
    // legacy font has neither descriptor nor BaseFont — use fallback
  } else throw e;
}

When it happens

Trigger: A simple (non-Type0, non-Type3) font dictionary with no /FontDescriptor and a /BaseFont that is missing, null, a string, or any non-Name object. Fires during translateFont() in the font-load path.

Common situations: Older or minimalist PDF producers that omit FontDescriptor and also fail to declare BaseFont as a Name. Also seen in heavily stripped/subset PDFs where optimization removed the BaseFont. Rare; typically indicates a seriously malformed font dictionary.

Related errors

• Invalid entry in 'Differences' array: ${data}
• Encoding is not a Name nor a Dict
• Max size of CID is 65,535
• invalid font Subtype
• Descendant fonts are not specified

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/38758abc351900f1. Report an issue: GitHub.