@Override
  public PdxWriter writeUnreadFields(PdxUnreadFields unread) {
    if (fieldsWritten()) {
      throw new PdxFieldAlreadyExistsException(
          "writeUnreadFields must be called before any other fields are written.");
    }
    unreadData = (PdxUnreadData) unread;
    return this;
  }
Make sure we have room to add a VLF offset.
geode-core/src/main/java/org/apache/geode/pdx/internal/PdxWriterImpl.java