Empty subscribe on valueChanges in Angular2

I have this weird case, a subscribe that never fires if left empty.

This doesn't work:

this.formGroup.get('unitCount').valueChanges
.do(value => console.log(value))
.subscribe();

When this works fine:

this.formGroup.get('unitCount').valueChanges
.do(value => console.log(value))
.subscribe(() => true);

Here I used () => true, but it could anything, false, void 0, even an empty object {}

Why can't I leave the subscribe() empty?