docs: protected void addFieldInto(int[] values, DurationFieldType field, int value) {
          int index = indexOf(field);
          if (index == -1) {
              if (value != 0 || field == null) {
                  throw new IllegalArgumentException(
                      "Period does not support field '" + field + "'");
              }
          } else {
              values[index] = FieldUtils.safeAdd(values[index], value);
          }
      }