Why the merge operator is not returning the first observable that it receives?

My understanding is that I can use the merge operator with 2 other Observables followed by the firstOrError operator to return the first Observable that returns a result.

I'm trying to run a test case but it doesn't seem to do what I thought it would do, so not sure if there's an issue with my test or I'm using the operators incorrectly, I appreciate if anyone could help me with that. If anyone know of a better way to achieve the same results that would be great too.

Here's my test case:

@Test
public void testMergeOperator() {
    TestObserver<String> observer = new TestObserver<>();

    Observable<String> foo = Observable.just("FOO");
    Observable<String> bar = Observable.timer(2, SECONDS).just("BAR");

    foo.publish(first -> Observable.merge(first, bar.takeUntil(first))
            .firstOrError()
            .toObservable())
            .subscribe(observer);

    assertEquals(1, observer.valueCount());
    assertEquals("FOO", observer.values().get(0));
}

And I'm getting:

junit.framework.ComparisonFailure: Expected :FOO Actual :BAR