{"id":"f7ba542335cb6250","repo":"gofiber/fiber","slug":"route-handler-fn-cannot-be-nil","errorCode":null,"errorMessage":"route handler 'fn' cannot be nil","messagePattern":"route handler 'fn' cannot be nil","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"app.go","lineNumber":1200,"sourceCode":"\t}\n}\n\n// RouteChain creates a Registering instance that lets you declare a stack of\n// handlers for the same route. Handlers defined via the returned Register are\n// scoped to the provided path.\nfunc (app *App) RouteChain(path string) Register {\n\t// Create new route\n\troute := &Registering{app: app, path: path}\n\n\treturn route\n}\n\n// Route is used to define routes with a common prefix inside the supplied\n// function. It mirrors the legacy helper and reuses the Group method to create\n// a sub-router.\nfunc (app *App) Route(prefix string, fn func(router Router), name ...string) Router {\n\tif fn == nil {\n\t\tpanic(\"route handler 'fn' cannot be nil\")\n\t}\n\t// Create new group\n\tgroup := app.Group(prefix)\n\tif len(name) > 0 {\n\t\tgroup.Name(name[0])\n\t}\n\n\t// Define routes\n\tfn(group)\n\n\treturn group\n}\n\n// Error makes it compatible with the `error` interface.\nfunc (e *Error) Error() string {\n\treturn e.Message\n}\n","sourceCodeStart":1182,"sourceCodeEnd":1218,"githubUrl":"https://github.com/gofiber/fiber/blob/9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c/app.go#L1182-L1218","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","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."],"exampleFix":"// before: nil function passed\nvar register func(fiber.Router)\napp.Route(\"/api\", register)\n\n// after: ensure non-nil\nif register != nil {\n    app.Route(\"/api\", register)\n}\n// or always define it:\napp.Route(\"/api\", func(r fiber.Router) {\n    r.Get(\"/users\", getUsers)\n})","handlingStrategy":"validation","validationCode":"if register != nil {\n    app.Route(\"/api\", register)\n}","typeGuard":"func isRouteFn(fn func(fiber.Router)) bool { return fn != nil }","tryCatchPattern":null,"preventionTips":["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."],"tags":["routing","route-group","nil-check","panic","config"],"analyzedSha":"9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c","analyzedAt":"2026-08-04T21:44:03.395Z","schemaVersion":2}