"SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON" when querying custom Admin endpoint

[AlexTechNoir]

Hello!

I wanted to test an authenticated request to the custom Admin endpoint that I've created:

  router.options("/admin/downloadable_assets", cors(adminCorsOptions))
  router.get("/admin/downloadable_assets", cors(adminCorsOptions), authenticate(), (req, res) => {

    if (!req.user) {
      res.status(401).json({ message: "Not authorized" })
    }

    res.json({ data: "data" })
  })

I have a fetching function in Medusa Admin:

    async function fetchUrls() {
      const data = await fetch(
        "http://localhost:9000/admin/downloadable_assets",
        {
          credentials: "include",
        }
      )
        .then((response) => response.json())
        .catch((err) => console.error("Err: ", err))

      console.log(data)
    }
    fetchUrls()

The error was caught in the console: Err: SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

I run npm run build and medusa migrations run, but that didn't help

Can anyone tell me what I am doing wrong?