why i'm getting "_debugSource" as "undefined" in FiberNode?

Question

My question is why I'm getting _debugSource as undefined in FiberNode?

import * as React from "react";

export default function SimplePopper() {
  const handleClick = (event: React.MouseEvent<HTMLElement>) => {
    console.log(
      event.currentTarget[
        Object.keys(event.currentTarget).reduce((key) => {
          if (key.match(new RegExp("__reactFiber", "i"))) {
            return key;
          }
          return null;
        })
      ]
    );
  };

  return (
    <div>
      <button type="button" onClick={handleClick}>
        button
      </button>
    </div>
  );
}

In this code sample first I'm getting the DOM reference of the Button and then extracting fiberNode from that reference object

As you can see in this image, its undefined