Word Online cannot open .docx files generated with docx@9.5.1 (works fine in v8.1.3)

[EedupugantiVenkatesh]

Summary

When generating .docx files using docx@9.5.1, the documents open perfectly in Neat Office and LibreOffice,
but fail to open in Microsoft Word Online and sometimes in desktop Word (Windows) with the error:

"We're sorry. We can't open this document because of an error in its contents."

Downgrading to docx@8.1.3 makes the exact same code and document structure work perfectly everywhere.

Environment

• Node.js: v18.x / v20.x
• OS: Windows
• docx version: 9.5.1
• Also tested on: 8.1.3 (works fine)

Reproduction

Minimal example:

const { Document, Packer, Paragraph, TextRun, Footer, AlignmentType, PageNumber } = require("docx");

const doc = new Document({
  sections: [{
    properties: {},
    headers: {
      default: new Footer({
        children: [
          new Paragraph({
            alignment: AlignmentType.RIGHT,
            children: [new TextRun({ children: ["Page ", PageNumber.CURRENT] })],
          }),
        ],
      }),
    },
    children: [new Paragraph("Hello world")],
  }],
});

Packer.toBuffer(doc).then(buffer => require("fs").writeFileSync("test.docx", buffer));


Result

✅ Works fine in Neat Office / LibreOffice.

❌ Word Online shows "Error in contents".

❌ Word Desktop prompts to repair.

Expected

Should open in all versions of Microsoft Word.