What causes "failed to convert: %w" in gofiber/fiber?

Provide a default to make Convert tolerant: fiber.Convert(v, strconv.Atoi, 0).

failed to convert

Error message

failed to convert: %w

What it means

Returned by the generic helpers.Convert[T] function when the supplied converter returns an error AND no default value was provided. The converter's error is wrapped as 'failed to convert: <err>'. If a default value is passed (variadic defaultValue), Convert swallows the error and returns the default instead, so this error only escapes when the caller did not supply a fallback.

Solutions

Provide a default to make Convert tolerant: fiber.Convert(v, strconv.Atoi, 0).
Validate/trim the input string before calling Convert (strings.TrimSpace, format check).
Switch to an explicit parser with a clear error if you need to distinguish 'missing' from 'malformed'.
At system boundaries (config load), surface Convert errors to fail fast at startup rather than at request time.

Example fix

// before
port, err := fiber.Convert(os.Getenv("PORT"), strconv.Atoi)

// after
port, err := fiber.Convert(os.Getenv("PORT"), strconv.Atoi, 8080)

Defensive patterns

Strategy: validation

Validate before calling

// validate the string parses before calling Convert when you need to fail loudly
if _, err := strconv.Atoi(strings.TrimSpace(value)); err != nil {
    return fmt.Errorf("invalid integer %q: %w", value, err)
}
// or pass a default so Convert never returns this error at all:
v, _ := fiber.Convert(value, strconv.Atoi, 0)

Type guard

null

Try / catch

v, err := fiber.Convert(value, strconv.Atoi)
if err != nil {
    // converter failed and no default supplied
    return fmt.Errorf("bad value %q: %w", value, err)
}

Prevention

Pass a defaultValue to Convert when a fallback is acceptable.
Trim whitespace and validate format before Convert when you must distinguish missing vs malformed.
At config/env load time, fail fast on Convert errors instead of deferring to request time.
Reserve the no-default Convert call for inputs whose malformed state is itself a hard error.

When it happens

Trigger: Calling fiber.Convert(value, strconv.Atoi) (no default) where value is not an integer; fiber.Convert(s, strconv.ParseBool) on a non-bool; or any Convert with a custom converter that errors and no defaultValue argument.

Common situations: Parsing query/path/header values into typed fields without validation; a missing or malformed env var parsed via Convert; configuration loaded from strings where a value is blank or has stray whitespace; version bumps that change the expected format.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/c8b32ddffb2b4f8c. Report an issue: GitHub.

Appendix: source

Thrown at helpers.go:1234

	}

	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

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