sharp: Vector file to PNG doesn't preserve transparent alpha channel

The output is an image on a white background. I’ve tried various combinations of embed() and background({a: 0}) to no effect.

    let transform = (width, height, density, toFormat) => {
        return sharp(null, {density: density})
            .resize(width, height)
            .max()
            .withoutEnlargement()
            .toFormat(toFormat);
    };