Code that does not compile with Svelte 3.8.0 and Typescript 3.5.3

preprocess-sass.ts

import * as sass from 'node-sass';
import * as svelte from 'svelte/compiler';
import * as fs from 'fs';
import { promisify } from 'util';

const renderSass = promisify(sass.render);
const readFile = promisify(fs.readFile);

const preprocessSass = async (data: string) => {
	const result = await renderSass({ data });
	return {
		code: result.css.toString(),
	};
};

export const load = async (fileName: string) => {
	const source = await readFile(fileName).then(buffer => buffer.toString());
	const preprocessedSource = await svelte.preprocess(source, {
		// compilation fails here
		style: (input) => preprocessSass(input.content),
	});
	return svelte.parse(preprocessedSource.code);
};