skipUntil - v6.1.0 - throws an exception 'unsubscribe' of undefined

[claylaut]

RxJS version: 6.1.0 / 6.0.0 (tested on both)

Code to reproduce:
https://stackblitz.com/edit/rxjs-skipuntil-issue

Expected behaviour:
it should work without throwing an exception

Actual behaviour:
Throwing an exception:
Error in rxjs@6.1.0/internal/operators/skipUntil.js (60:32)
Cannot read property 'unsubscribe' of undefined

Additional information:
This issue is related to this change:
https://github.com/ReactiveX/rxjs/pull/3512/files#diff-fb87e1b2c0661f48fca4bf43fb5a19a3R60

PR including Fix
#3686

This works perfectly in 5.5.10 and lower versions.