How to use RxJS 5 buffer function? - rxjs5

Question

I googled a lot, tried to look at d.ts file but still cannot understand what is wrong. Cannot find example of RxJs5 how to use this function.
var source = Observable.fromEvent(document.body, 'keypress');
var delayedSource = source.delay(1000);
var obs = source
.buffer(() => {
return delayedSource;
})
.map((clickBuffer) => {
return clickBuffer.length;
});
I am getting error:
Error:(166, 15) TS2345: Argument of type '() => Observable<{}>' is not assignable to parameter of type 'Observable'.
Property '_isScalar' is missing in type '() => Observable<{}>'.
buffer.d.ts looks this, I guess I should understand from this, but I can't.
import { Observable } from '../Observable';
/**
* Buffers the incoming observable values until the passed `closingNotifier`
* emits a value, at which point it emits the buffer on the returned observable
* and starts a new buffer internally, awaiting the next time `closingNotifier`
* emits.
*
* <img src="./img/buffer.png" width="100%">
*
* #param {Observable<any>} closingNotifier an Observable that signals the
* buffer to be emitted} from the returned observable.
* #returns {Observable<T[]>} an Observable of buffers, which are arrays of
* values.
*/
export declare function buffer<T>(closingNotifier: Observable<any>): Observable<T[]>;
export interface BufferSignature<T> {
(closingNotifier: Observable<any>): Observable<T[]>;
}
This question came from this:
Counting keypresses per second with angular 2 Rxjs

Answer 1

Based on Benjamin Gruenbaum comment, this solves the problem:
var source = Observable.fromEvent(document.body, 'keypress');
var delayedSource = source.delay(1000);
var obs = source
.buffer(delayedSource)
.map((clickBuffer) => {
return clickBuffer.length;
})