Rx bug?

gistfile1.txt

var subject = new Rx.Subject();
var getObservable = function (eventName) {
    return subject.asObservable()
        .where(function (x) { return x.EventName === eventName; })
        .takeUntil(function (x) { return x.Type === "onComplete"; })
        .selectMany(function (x) {
            return x.Type === "onNext" ? Rx.Observable.returnValue(x.Data) :
                x.Type === "onError" ? Rx.Observable.throwException(x.Data) :
                    Rx.Observable.empty();
        });


}

getObservable("SomethingHappened").subscribe(function(x){ console.log("onNext" + x); }, function(x){ console.log("onError" + x); }, function(){ console.log("onComplete");})

This throws:

TypeError: Object function (x) { return x.Type === "onComplete"; } has no method 'materialize'

as soon as I put in the takeUntil. If I leave it out, everything works as expected.