What causes "route handler 'fn' cannot be nil" in gofiber/fiber?

Pass a non-nil func(router fiber.Router): app.Route("/api", func(r fiber.Router) { r.Get("/x", h) }).

route handler 'fn' cannot be nil

Error message

route handler 'fn' cannot be nil

What it means

Panicked by app.Route (app.go:1200) when the callback function `fn` passed to Route(prefix, fn, name...) is nil. Route is a helper that creates a Group and invokes fn(group) to register sub-routes; a nil fn would panic on call, so Fiber pre-validates and fails fast with a clear message.

Source

Thrown at app.go:1200

	}
}

// RouteChain creates a Registering instance that lets you declare a stack of
// handlers for the same route. Handlers defined via the returned Register are
// scoped to the provided path.
func (app *App) RouteChain(path string) Register {
	// Create new route
	route := &Registering{app: app, path: path}

	return route
}

// Route is used to define routes with a common prefix inside the supplied
// function. It mirrors the legacy helper and reuses the Group method to create
// a sub-router.
func (app *App) Route(prefix string, fn func(router Router), name ...string) Router {
	if fn == nil {
		panic("route handler 'fn' cannot be nil")
	}
	// Create new group
	group := app.Group(prefix)
	if len(name) > 0 {
		group.Name(name[0])
	}

	// Define routes
	fn(group)

	return group
}

// Error makes it compatible with the `error` interface.
func (e *Error) Error() string {
	return e.Message
}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Pass a non-nil func(router fiber.Router): app.Route("/api", func(r fiber.Router) { r.Get("/x", h) }).
Guard the call: if routesFn != nil { app.Route("/api", routesFn) }.
Initialize the route function at declaration, not conditionally.

Example fix

// before: nil function passed
var register func(fiber.Router)
app.Route("/api", register)

// after: ensure non-nil
if register != nil {
    app.Route("/api", register)
}
// or always define it:
app.Route("/api", func(r fiber.Router) {
    r.Get("/users", getUsers)
})

Defensive patterns

Strategy: validation

Validate before calling

if register != nil {
    app.Route("/api", register)
}

Type guard

func isRouteFn(fn func(fiber.Router)) bool { return fn != nil }

Prevention

Always pass a non-nil func(fiber.Router) to app.Route.
Initialize route-registration functions at declaration.
Guard conditionally-defined route functions before calling Route.

When it happens

Trigger: Calling app.Route("/api", nil) — typically because a route-registration function variable was not initialized, a conditional left the function nil, or a refactor removed the function body leaving a nil reference.

Common situations: Conditional route registration where the sub-router function is conditionally nil; refactoring that extracted route logic into a variable that was never assigned; copy-paste forgetting the closure.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/f7ba542335cb6250.json. Report an issue: GitHub.