{"record":{"id":"5ce77798779643ae","repo":"gofiber/fiber","slug":"s-invalid-handler-d-t","errorCode":null,"errorMessage":"%s: invalid handler #%d (%T)\n","messagePattern":"(.+?): invalid handler #(.+?) \\(%T\\)\n","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"adapter.go","lineNumber":275,"sourceCode":"\tadapted := fasthttpadaptor.NewFastHTTPHandler(handler)\n\n\treturn func(c Ctx) error {\n\t\tadapted(c.RequestCtx())\n\t\treturn nil\n\t}\n}\n\n// collectHandlers converts a slice of handler arguments to Fiber handlers.\n// The context string is used to provide informative panic messages when an\n// unsupported handler type is encountered.\nfunc collectHandlers(context string, args ...any) []Handler {\n\thandlers := make([]Handler, 0, len(args))\n\n\tfor i, arg := range args {\n\t\thandler, ok := toFiberHandler(arg)\n\n\t\tif !ok {\n\t\t\tpanic(fmt.Sprintf(\"%s: invalid handler #%d (%T)\\n\", context, i, arg))\n\t\t}\n\t\thandlers = append(handlers, handler)\n\t}\n\n\treturn handlers\n}\n","sourceCodeStart":257,"sourceCodeEnd":282,"githubUrl":"https://github.com/gofiber/fiber/blob/a105acad6c1e4576a77f01e02973f67e962bb58d/adapter.go#L257-L282","documentation":"collectHandlers converts variadic args (passed to Get/Post/Use/etc.) into Fiber Handler values via toFiberHandler. If an argument is none of the accepted handler shapes (func(*Ctx) error, middleware, []Handler, etc.) it panics with the offending index and Go type. This is a programming error caught at route-registration time, before the server starts.","triggerScenarios":"Passing an int, struct, nil non-handler, or a function with the wrong signature to app.Get/Post/Put/etc. or to a Group/Use call. The panic message shows the 0-based argument index and the dynamic type.","commonSituations":"Passing a method value with the wrong receiver type; passing a handler builder's result that returned nil; passing a value of a custom type that looks like a handler but is not func(*fiber.Ctx) error; refactoring that changes a function signature.","solutions":["Look at the panic's argument index (#%d) and check that positional argument in the route-registration call.","Ensure every handler is exactly func(*fiber.Ctx) error (or a type Fiber accepts via toFiberHandler).","If passing a method, bind it as a closure: func(c *fiber.Ctx) error { return svc.method(c) }.","Remove nil entries from handler slices before passing them."],"exampleFix":"// before\napp.Get(\"/u\", userController, userHandler)\n// where userController is a *UserController, not a handler\n\n// after\napp.Get(\"/u\", func(c *fiber.Ctx) error { return userController.handle(c) })","handlingStrategy":"type-guard","validationCode":"// Validate handler args before passing them to Get/Post/Use\nfunc toFiberHandlerSafe(arg any) (fiber.Handler, error) {\n    switch h := arg.(type) {\n    case fiber.Handler:\n        return h, nil\n    case func(*fiber.Ctx) error:\n        return h, nil\n    case []fiber.Handler:\n        // flatten elsewhere; reject here\n        return nil, fmt.Errorf(\"unexpected []Handler\")\n    }\n    return nil, fmt.Errorf(\"not a handler: %T\", arg)\n}","typeGuard":"func isFiberHandler(v any) bool {\n    switch v.(type) {\n    case fiber.Handler, func(*fiber.Ctx) error:\n        return true\n    }\n    return false\n}","tryCatchPattern":"// Defer-recover around route registration in tests to surface handler-type mistakes\nfunc registerSafely(app *fiber.App, method, path string, args ...any) (err error) {\n    defer func() {\n        if r := recover(); r != nil {\n            err = fmt.Errorf(\"register %s %s: %v\", method, path, r)\n        }\n    }()\n    app.Add(method, path, args...)\n    return nil\n}","preventionTips":["Standardize on func(*fiber.Ctx) error closures for all handlers.","Lint handler slices for nil before route registration.","Add a unit test that boots the app to catch registration panics during CI."],"tags":["routing","handler","type-mismatch","startup"],"backgroundTag":null,"analyzedSha":"a105acad6c1e4576a77f01e02973f67e962bb58d","analyzedAt":"2026-08-11T17:33:26.942Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-14T00:17:10.932Z"}