code: private long calculateWeight(final Entry e, final Object v) {
      long _weight;
      if (v instanceof ExceptionWrapper) {
        _weight = 1;
      } else {
        _weight = weigher.weigh(e.getKey(), v);
      }
      if (_weight < 0) {
        throw new IllegalArgumentException("weight must be positive.");
      }
      return _weight;
    }
docs: Exceptions have the minimum weight.