public KnownVersion getClientServerProtocolVersion() {
    for (short i = ordinal(); i >= 0; i--) {
      if (VALUES[i] != null && VALUES[i].modifiesClientServerProtocol) {
        return VALUES[i];
      }
    }
    throw new IllegalStateException("There is no valid clientServerProtocolVersion for " + this);
  }
an invalid KnownVersion used for tests and instead of null values
geode-serialization/src/main/java/org/apache/geode/internal/serialization/KnownVersion.java