gofiber/fiber · error · ErrCustomBinderNotFound

binder: custom binder not found, please be sure to enter the

Error message

binder: custom binder not found, please be sure to enter the right name

What it means

ErrCustomBinderNotFound (error.go:51) is returned by Bind().Custom(name, dest) (bind.go:236) when no custom binder registered via app.RegisterCustomBinder matches the requested name. Custom binders are looked up by their Name() method; a typo or an unregistered binder yields this error so the caller knows binding was never attempted.

Source

Thrown at error.go:51

var (
	// ErrRangeMalformed is returned for a syntactically invalid Range header,
	// which RFC 9110 Section 14.2 allows a server to reject; it carries a
	// 400 Bad Request status so propagating it does not surface as a 500.
	ErrRangeMalformed = NewError(StatusBadRequest, "range: malformed range header string")
	// ErrRangeUnsupported is returned for a Range header whose range unit is
	// not "bytes". RFC 9110 Section 14.2 requires an origin server to IGNORE
	// a Range header field with a range unit it does not understand, so
	// callers receiving this error should serve the full representation
	// instead of returning an error response. It still carries a
	// 400 Bad Request status as a safety net, so blind propagation does not
	// surface as a 500.
	ErrRangeUnsupported   = NewError(StatusBadRequest, "range: unsupported range unit")
	ErrRangeTooLarge      = NewError(StatusRequestedRangeNotSatisfiable, "range: too many ranges")
	ErrRangeUnsatisfiable = errors.New("range: unsatisfiable range")
)

// Binder errors
var ErrCustomBinderNotFound = errors.New("binder: custom binder not found, please be sure to enter the right name")

// Format errors
var (
	// ErrNoHandlers is returned when c.Format is called with no arguments.
	ErrNoHandlers = errors.New("format: at least one handler is required, but none were set")
)

// gofiber/schema errors
type (
	// ConversionError Conversion error exposes the internal schema.ConversionError for public use.
	ConversionError = schema.ConversionError
	// UnknownKeyError error exposes the internal schema.UnknownKeyError for public use.
	UnknownKeyError = schema.UnknownKeyError
	// EmptyFieldError error exposes the internal schema.EmptyFieldError for public use.
	EmptyFieldError = schema.EmptyFieldError
	// MultiError error exposes the internal schema.MultiError for public use.
	MultiError = schema.MultiError
)

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Verify the name passed to Custom() exactly matches the Name() returned by the registered binder.
2. Register the custom binder during app setup with app.RegisterCustomBinder(myBinder) before serving requests.
3. Centralize binder names as shared constants to avoid string drift.

Example fix

// before
app.RegisterCustomBinder(myBinder) // myBinder.Name() == "formBody"
c.Bind().Custom("formbody", &out) // wrong case -> not found

// after
const BinderFormBody = "formBody"
app.RegisterCustomBinder(myBinder) // Name() returns BinderFormBody
c.Bind().Custom(BinderFormBody, &out)

// Share binder-name constants and verify registration at startup.
const BinderJSON = "json"
found := false
for _, b := range app.CustomBinders() {
    if b.Name() == BinderJSON { found = true; break }
}
if !found { log.Fatalf("custom binder %q not registered", BinderJSON) }

if err := c.Bind().Custom(BinderJSON, &out); err != nil {
    if errors.Is(err, fiber.ErrCustomBinderNotFound) {
        return fiber.NewError(fiber.StatusInternalServerError, "binder misconfigured")
    }
    return err
}

When it happens

Trigger: Calling c.Bind().Custom("jsonBody", &out) when the binder was registered under a different name (e.g. "body"), or calling Custom before RegisterCustomBinder ran. Also when a binder is conditionally registered and the request hits a code path where it is absent.

Common situations: Name mismatches between registration and call sites, registration in an init() that didn't run, or refactoring that renamed a binder without updating all callers.

Related errors

• min constraint requires an argument
• max constraint requires an argument
• range constraint requires two arguments
• regex constraint requires a pattern argument
• fiber: no view engine configured

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/257591d2b8dda296.json. Report an issue: GitHub.