{
  // A value the conversion cannot read at all, here because listing its keys
  // throws, is represented by a marker carrying the error's message.
  "keys throw": new Proxy({}, {
    ownKeys() {
      throw new Error("no keys");
    },
  }),
}

--

keys throw:
/unconvertible("no keys")
/unconvertible("no keys")
{
  "/unconvertible": "no keys"
}