{"record":{"id":"1f4d33d6d4962a89","repo":"gofiber/fiber","slug":"route-handler-fn-cannot-be-nil-1f4d33","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":"group.go","lineNumber":243,"sourceCode":"\t\tmatcher: parseDomainPattern(host),\n\t}\n}\n\n// RouteChain creates a Registering instance scoped to the group's prefix,\n// allowing chained route declarations for the same path.\nfunc (grp *Group) RouteChain(path string) Register {\n\t// Create new group\n\tregister := &Registering{app: grp.app, group: grp, path: getGroupPath(grp.Prefix, path)}\n\n\treturn register\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 (grp *Group) 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 := grp.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","sourceCodeStart":225,"sourceCodeEnd":256,"githubUrl":"https://github.com/gofiber/fiber/blob/a105acad6c1e4576a77f01e02973f67e962bb58d/group.go#L225-L256","documentation":"Group.Route(prefix, fn, name...) requires a non-nil callback that registers routes inside the newly created sub-group. A nil fn is rejected up front because silently building an empty group would mask a bug in the caller's route-registration logic. The check precedes group construction.","triggerScenarios":"Calling grp.Route(\"/v1\", nil) — the callback is nil because it was conditionally assigned, came from an unset variable, or was passed through from a wrapper that did not validate it.","commonSituations":"Conditional route registration (var fn func(fiber.Router); if flag { fn = ... }; grp.Route(prefix, fn)) hitting the unset branch; refactoring that leaves an unresolved callback; calling Route in a loop where some iterations have nothing to register.","solutions":["Always pass a non-nil func literal: grp.Route(prefix, func(r fiber.Router) { ... }).","Guard the call site when the callback may be absent: if fn != nil { grp.Route(prefix, fn) }.","Default uninitialized route-builder variables to func(fiber.Router) {} instead of nil."],"exampleFix":"// before\nvar fn func(router Router)\nif enableAdmin { fn = adminRoutes }\ngrp.Route(\"/admin\", fn)\n// after\nif fn != nil {\n    grp.Route(\"/admin\", fn)\n}","handlingStrategy":"validation","validationCode":"if routeBuilder == nil {\n    return errors.New(\"route builder callback must not be nil\")\n}\ngrp.Route(\"/v1\", routeBuilder)","typeGuard":null,"tryCatchPattern":"defer func() {\n    if r := recover(); r != nil {\n        log.Fatalf(\"Route fn nil: %v\", r)\n    }\n}()\ngrp.Route(\"/v1\", fn)","preventionTips":["Initialize route-builder variables to func(Router) {} instead of nil.","Always pass a func literal when possible.","Add a nil check at any wrapper that forwards a callback to Route."],"tags":["routing","group","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"}