pump stream continues after callback

output

lilith[4835]$ node pb.js
pump cb 2 0 [Error: second call, carrying on]
ws finished 4 49152

pb.js

'use strict';

const pump = require('pump');
const stream = require('stream');

class Readable extends stream.Readable {
  constructor() {
    super();
    this.called = 0;
  }
  _read(size) {
    let chunk = new Buffer(size);
    chunk.fill(0);
    this.called += 1;
    if (2 === this.called) {
      this.emit('error', new Error('second call, carrying on'));
    } else if (4 <= this.called) {
      chunk = null;
    }
    return this.push(chunk);
  }
}

class Writable extends stream.Writable {
  constructor() {
    super();
    this.received = 0;
  }
  _write(chunk, encoding, callback) {
    this.received += chunk.length;
    return callback();
  }
}

const rs = new Readable();
const ws = new Writable();
ws.on('finish', () => {
  console.log('ws finished', rs.called, ws.received);
});
pump(rs, ws, err => {
  console.log('pump cb', rs.called, ws.received, err);
});