Using go 1.21.4 windows/adm64

I’m experiencing an interesting issue when trying to zip files using go’s archive/zip library.
Files are all written to the archive, but Windows reports that the zip file is invalid. I can open the achive in 7-Zip, and it shows a “Packed Size” but no “Size” for the last file added to the archive, and a CRC of 00000000 for that file.

Is it possible that the archive file is being closed before the last file has been completely written?

func zipFiles(clipFolder string, zipName string, files []string) int {
    archive, err := os.Create(zipName)
    if err != nil {
        _ = elog.Errorf("Error creating %s: %s", zipName, err.Error())
        return 0
    }
    defer archive.Close()
    writer := zip.NewWriter(archive)

    zipped := 0
    for _, file := range files {
            zipped += writeFileToArchive(writer, clipFolder, file)
        }
    }
    return zipped
}

// writeFileToArchive adds a file to the zip archive and returns the number of files added (0 or 1)
func writeFileToArchive(writer *zip.Writer, path string, filename string) int {
    w, err := writer.Create(filename)
    if err != nil {
        elog.Warnf("could not add %s to archive", filename)
        return 0
    }

    fullPath := filepath.Join(path, filename)
    source, err := os.Open(fullPath)
    if err != nil {
        elog.Warnf("could not open %s file for reading", fullPath)
        return 0
    }
    defer source.Close()
    _, err = io.Copy(w, source)
    if err != nil {
        elog.Warnf("could not copy %s into archive", filename)
        return 0
    }
    _ = writer.Flush()

    return 1
}

I added a call to writer.Flush() after adding each file to the archive, but that doesn’t help. And I’ve tried writing various numbers of files – it’s always the last file that isn’t being handled properly.