public static Iterable<? extends KnownVersion> getAllVersions() {
    return Arrays.stream(VALUES).filter(x -> x != null && x != TEST_VERSION)
        .collect(Collectors.toList());
  }
an invalid KnownVersion used for tests and instead of null values
geode-serialization/src/main/java/org/apache/geode/internal/serialization/KnownVersion.java