Graph Inference: bad graph calculation leads to assertion error

An assertion is thrown when compiling this program:

interface Predicate<X> {
    boolean accepts(X x);
}

class Optional<T> {
    public boolean isPresent() { return false; }
    public static<E> Optional<E> empty() { return null; }
}

interface Supplier<X> {
    X make();
}

class FindSink<O, T> { }

class OfRef<R> extends FindSink<R, Optional<R>> { }

class FindOp<T, O> {
    public static<F> FindOp<F, Optional<F>> makeRef() {
        return new FindOp<>(Optional.empty(), Optional::isPresent, OfRef::new);
    }
 
    private FindOp(O emptyValue,
                   Predicate<O> presentPredicate,
                   Supplier<FindSink<T, O>> sinkSupplier) {
    }
}