未捕获的TypeError：RxJS订阅块中的事件目标无效

Question

有人可以告诉我这段代码有什么问题吗？

const field = document.querySelector('.form-field');
const showHistoryButton = document.querySelector('#show-history');
const historyPanel = document.querySelector('#history');

const showHistory$ = Rx.Observable.fromEvent(showHistoryButton, 'click');

Rx.Observable.fromEvent(field, 'keyup')
    .debounceTime(200)
    .pluck('target', 'value')
    .filter(R.compose(R.not, R.isEmpty))
    .bufferWhen(() => showHistory$)
    .do(history => history.pop())
    .subscribe(history => {
        let contents = '';
        if(history.length > 0) {
            for(let item of history) {
                contents += '<li>' + item + '</li>';
            }
            historyPanel.innerHTML = contents;
        }
    });

我得到的错误是：

rxjs.5.5.2.js:3895 Uncaught TypeError: Invalid event target
    at Function.FromEventObservable.setupSubscription (rxjs.5.5.2.js:3895)
    at FromEventObservable._subscribe (rxjs.5.5.2.js:3917)
    at FromEventObservable.Observable._trySubscribe (rxjs.5.5.2.js:825)
    at FromEventObservable.Observable.subscribe (rxjs.5.5.2.js:813)
    at subscribeToResult (rxjs.5.5.2.js:2065)
    at BufferWhenSubscriber.openBuffer (rxjs.5.5.2.js:7489)
    at new BufferWhenSubscriber (rxjs.5.5.2.js:7442)
    at BufferWhenOperator.call (rxjs.5.5.2.js:7427)
    at Observable.subscribe (rxjs.5.5.2.js:810)
    at DoOperator.call (rxjs.5.5.2.js:9862)

当我取消注释代码片段时，即使它是空的，看起来罪魁祸首也是subscribe方法。我使用的是RxJS 5.5.2，唯一的依赖是Ramda。