gofiber/fiber · warning

failed to convert: %w

Error message

failed to convert: %w

What it means

Returned by the generic fiber.Convert[T] helper when the supplied converter function fails AND no default value was provided. Convert wraps any converter (strconv.Atoi, time.Parse, etc.) so callers get a uniform error. If a default value is passed, Convert returns it instead of erroring.

Source

Thrown at helpers.go:1294

	}

	switch m {
	case MethodPut, MethodDelete:
		return true
	default:
		return false
	}
}

// Convert a string value to a specified type, handling errors and optional default values.
func Convert[T any](value string, converter func(string) (T, error), defaultValue ...T) (T, error) {
	converted, err := converter(value)
	if err != nil {
		if len(defaultValue) > 0 {
			return defaultValue[0], nil
		}

		return converted, fmt.Errorf("failed to convert: %w", err)
	}

	return converted, nil
}

var (
	errParsedEmptyString = errors.New("parsed result is empty string")
	errParsedEmptyBytes  = errors.New("parsed result is empty bytes")
	errParsedType        = errors.New("unsupported generic type")
	// errParseValue flags a failed numeric/bool parse; callers only test err != nil.
	errParseValue = errors.New("failed to parse value")
)

// genericParseType parses str into V. Parse failures return the static errParseValue
// sentinel: the error is never surfaced (callers only test err != nil), so a flat
// sentinel is enough and avoids a per-call fmt.Errorf alloc on the hot default path.
func genericParseType[V GenericType](str string) (V, error) {
	var v V

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Pass a default value as the variadic arg: fiber.Convert(value, strconv.Atoi, 0).
2. Validate the input string format before calling Convert.
3. Inspect the wrapped error to give a precise error message to the caller.
4. Use errors.Is/As on the wrapped error to branch on the underlying parse failure.

Example fix

// before
n, err := fiber.Convert(c.Query("page"), strconv.Atoi)

// after
n, err := fiber.Convert(c.Query("page"), strconv.Atoi, 1)
// or validate first:
// if !regexp.MustCompile(`^\d+$`).MatchString(c.Query("page")) { ... }

// Guard conversion with a default so it never errors
n, _ := fiber.Convert(c.Query("page"), strconv.Atoi, 1)

n, err := fiber.Convert(c.Query("page"), strconv.Atoi)
if err != nil {
    return c.Status(fiber.StatusBadRequest).SendString("invalid page")
}

When it happens

Trigger: Calling fiber.Convert(value, strconv.Atoi) where 'value' is non-numeric and no default is given, e.g. fiber.Convert("abc", strconv.Atoi). The converter returns an error which Convert re-wraps as 'failed to convert: %w'.

Common situations: Parsing query params or form fields without prior validation, a missing default for optional numeric params, or a format mismatch (e.g. time.Parse with the wrong layout).

Related errors

• log: context tag name and function are required
• logger: tag name and function are required
• proxy: upstream host is empty or invalid
• session ID cannot be empty
• field must not contain CR or LF

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/c8b32ddffb2b4f8c.json. Report an issue: GitHub.