{"record":{"id":"6527bacf6250f375","repo":"gofiber/fiber","slug":"route-handler-fn-cannot-be-nil-6527ba","errorCode":null,"errorMessage":"route handler 'fn' cannot be nil","messagePattern":"route handler 'fn' cannot be nil","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"error","filePath":"domain.go","lineNumber":594,"sourceCode":"\t\tapp:     d.app,\n\t\tgroup:   newGrp,\n\t\tmatcher: d.matcher,\n\t}\n}\n\n// RouteChain creates a Registering instance for the domain router.\nfunc (d *domainRouter) RouteChain(path string) Register {\n\treturn &domainRegistering{\n\t\tdomain: d,\n\t\tpath:   d.registerPath(path),\n\t}\n}\n\n// Route defines routes with a common prefix inside the supplied function,\n// scoped to the domain pattern.\nfunc (d *domainRouter) 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\n\tgroup := d.Group(prefix)\n\tif len(name) > 0 {\n\t\tgroup.Name(name[0])\n\t}\n\n\tfn(group)\n\n\treturn group\n}\n\n// Name assigns a name to the most recently registered route.\n// When the domain router was created from a Group, this delegates to the\n// group's Name method so that group name prefixes are applied correctly.\nfunc (d *domainRouter) Name(name string) Router {\n\tif d.group != nil {\n\t\td.group.Name(name)","sourceCodeStart":576,"sourceCodeEnd":612,"githubUrl":"https://github.com/gofiber/fiber/blob/a105acad6c1e4576a77f01e02973f67e962bb58d/domain.go#L576-L612","documentation":"domainRouter.Route(prefix, fn, name...) requires a non-nil callback fn that defines nested routes inside the group. Passing nil is a programming error with no valid use, so fiber panics immediately rather than silently creating an empty group. The guard runs before any group is constructed.","triggerScenarios":"Calling d.Route(\"/v1\", nil) on a domain router, typically because the callback variable was never assigned, was conditionally set to nil, or was passed by mistake from a higher-level wrapper.","commonSituations":"Conditionally building routes (var fn func(fiber.Router); if cond { fn = ... }; d.Route(prefix, fn)) and hitting the false branch; refactoring that leaves an unresolved symbol; calling Route inside a loop where one iteration has no routes to register.","solutions":["Ensure the callback is always a non-nil func literal: d.Route(prefix, func(r fiber.Router) { ... }).","If the callback may legitimately be absent, guard the call site: if fn != nil { d.Route(prefix, fn) }.","Initialize route-builder variables to a no-op func(fiber.Router) instead of nil when their assignment is conditional."],"exampleFix":"// before\nvar fn func(router Router)\nif registerV1 { fn = buildV1 }\nd.Route(\"/v1\", fn)\n// after\nif fn != nil {\n    d.Route(\"/v1\", fn)\n}","handlingStrategy":"validation","validationCode":"if routeBuilder == nil {\n    return errors.New(\"route builder callback must not be nil\")\n}\nd.Route(\"/v1\", routeBuilder)","typeGuard":null,"tryCatchPattern":"defer func() {\n    if r := recover(); r != nil {\n        log.Fatalf(\"Route fn nil: %v\", r)\n    }\n}()\nd.Route(\"/v1\", fn)","preventionTips":["Default route-builder variables to func(Router) {} rather than nil.","Guard conditional route registration with an explicit nil check at the call site.","Prefer passing func literals directly to Route instead of variables."],"tags":["routing","domain","nil-guard","startup"],"backgroundTag":null,"analyzedSha":"a105acad6c1e4576a77f01e02973f67e962bb58d","analyzedAt":"2026-08-11T17:33:26.942Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}