code: func InvalidEnumValueError(ctx string, val interface{}, allowed []interface{}) error {
  	elems := make([]string, len(allowed))
  	for i, a := range allowed {
  		elems[i] = fmt.Sprintf("%#v", a)
  	}
  	msg := fmt.Sprintf("value of %s must be one of %s but got value %#v", ctx, strings.Join(elems, ", "), val)
  	return ErrInvalidRequest(msg, "attribute", ctx, "value", val, "expected", strings.Join(elems, ", "))
  }
docs: // InvalidEnumValueError is the error produced when the value of a parameter or payload field does
  // not match one the values defined in the design Enum validation.