GloballyAsSingletonView is writing multiple values to the SingletonView

The code snippet below is writing multiple values to the sideInput. Resulting in exception: "PCollection with more than one element accessed as a singleton view."

PCollectionView<Integer> globalView = input
.apply(Window.<Integer>into(new GlobalWindows())
    .triggering(Repeatedly.forever(AfterPane.elementCountAtLeast(1)))
    .accumulatingFiredPanes())
.apply(Sum.integersGlobally().asSingletonView())