code: func InvalidAttributeTypeError(ctx string, val interface{}, expected string) error {
  	msg := fmt.Sprintf("type of %s must be %s but got value %#v", ctx, expected, val)
  	return ErrInvalidRequest(msg, "attribute", ctx, "value", val, "expected", expected)
  }
docs: // InvalidAttributeTypeError is the error produced when the type of payload field does not match
  // the type defined in the design.