"transform" option throw error for 'on' of undefined.

[ghost]

I have this:

...
var cpx = require("cpx");

async function copy() {
  return await cpx.copy(
    source,
    dest,
    { clean: true, transform: filepath => console.log(filepath) },
    err => {
      if (err) throw err;
      return console.log("OK");
    }
  );
}
...

and I receive this:

script/first.txt
script/second.txt

C:\scripts\script.js:113
      if (err) throw err;
               ^

TypeError: Cannot read property 'on' of undefined
    at C:\scripts\node_modules\cpx\lib\copy.js:56:15
    at Array.reduce (<anonymous>)
    at copyBody (C:\scripts\node_modules\cpx\lib\copy.js:54:28)
    at Object.action (C:\scripts\node_modules\cpx\lib\copy.js:119:13)
    at Immediate.dequeue [as _onImmediate] (C:\scripts\node_modules\cpx\lib\queue.js:37:10)
    at runCallback (timers.js:793:20)
    at tryOnImmediate (timers.js:751:5)
    at processImmediate [as _immediateCallback] (timers.js:722:5)
error Command failed with exit code 1.

As you can see it works good but at the end it throw error, why?

NodeJS 8.9.1, "cpx": "~1.5.0".