读取文件时出现类型错误

Question

const http = require("http");
const url = require("url");
const fs = require("fs");

http
  .createServer(function (req, res) {
    const pathname = url.parse(req.url, true).pathname;
    if (pathname == "/favicon.ico") return res.end();
    let filename;
    if (pathname == "/") filename = "./index.html";
    else filename = "." + pathname + ".html";
    console.log(filename);
    fs.readFile(filename, function (err, data) {
      if (err) {
        fs.readFile("./404.html", function (err, data) {
          res.writeHead(200, { "Content-Type": "text/html" });
          res.write(data);
          return res.end();
        });
      }
      res.writeHead(200, { "Content-Type": "text/html" });
      res.write(data);
      return res.end();
    });
  })
  .listen(8080, () => {
    console.log("Server made");
  });

每当我在 if(err) 条件下访问未指定的文件时，我都会尝试读取 404 html 文件。我得到的错误是 TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer or Uint8Array. Received undefined。为什么会出现这个错误？