code: public static boolean cannotBeValidatable(Object value) { // called by e.g. ResponseBeanValidator
          return value instanceof String // yes-yes-yes 
                  || value instanceof Number // e.g. Integer
                  || DfTypeUtil.isAnyLocalDate(value) // e.g. LocalDate
                  || value instanceof Boolean // of course
                  || value instanceof Classification // e.g. CDef
                  || value.getClass().isPrimitive() // probably no way, just in case
          ;
      }
docs: similar logic is on action response reflector