Unable to get config("mail.default"). Always return null

I'am trying to use SMTP in laravel 10. When a user registers, system will send verification email using MustVerifyEmail.

But I got this error

production.ERROR: Illuminate\Mail\MailManager::getConfig(): Argument #1 ($name) must be of type string, null given, called in /path/to/mywebsite/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php on line 111 

Then I traced to MailManager.php and found that null was passed to the mailer(). I didn't set MAIL_MAILER in env file. So supposed it shoud read the default value in config/mail.php

'default' => env('MAIL_MAILER', 'smtp'),

However, if I try to get the value by config('mail.default'). It always return null.

dd(config('mail.default')); // return null
dd(env('MAIL_MAILER', 'smtp')); // return "smtp"

I am stuck. Maybe somewhere is overriding the default value?

The following is the full error log. Please advise

production.ERROR: Illuminate\Mail\MailManager::getConfig(): Argument #1 ($name) must be of type string, null given, called in /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php on line 111 {"exception":"[object] (TypeError(code: 0): Illuminate\Mail\MailManager::getConfig(): Argument #1 ($name) must be of type string, null given, called in /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php on line 111 at /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php:445)
[stacktrace]
#0 /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php(111): Illuminate\Mail\MailManager->getConfig()
#1 /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php(98): Illuminate\Mail\MailManager->resolve()
#2 /website_path/vendor/laravel/framework/src/Illuminate/Mail/MailManager.php(76): Illuminate\Mail\MailManager->get()
#3 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/Channels/MailChannel.php(64): Illuminate\Mail\MailManager->mailer()
#4 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/NotificationSender.php(148): Illuminate\Notifications\Channels\MailChannel->send()
#5 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/NotificationSender.php(106): Illuminate\Notifications\NotificationSender->sendToNotifiable()
#6 /website_path/vendor/laravel/framework/src/Illuminate/Support/Traits/Localizable.php(19): Illuminate\Notifications\NotificationSender->Illuminate\Notifications\{closure}()
#7 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/NotificationSender.php(109): Illuminate\Notifications\NotificationSender->withLocale()
#8 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/NotificationSender.php(79): Illuminate\Notifications\NotificationSender->sendNow()
#9 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/ChannelManager.php(39): Illuminate\Notifications\NotificationSender->send()
#10 /website_path/vendor/laravel/framework/src/Illuminate/Notifications/RoutesNotifications.php(18): Illuminate\Notifications\ChannelManager->send()
#11 /website_path/vendor/laravel/framework/src/Illuminate/Auth/MustVerifyEmail.php(38): App\Models\User->notify()
#12 /website_path/vendor/laravel/framework/src/Illuminate/Auth/Listeners/SendEmailVerificationNotification.php(19): Illuminate\Foundation\Auth\User->sendEmailVerificationNotification()
#13 /website_path/vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php(441): Illuminate\Auth\Listeners\SendEmailVerificationNotification->handle()
#14 /website_path/vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php(249): Illuminate\Events\Dispatcher->Illuminate\Events\{closure}()
#15 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php(433): Illuminate\Events\Dispatcher->dispatch()
#16 /website_path/app/Http/Controllers/Auth/RegisteredUserController.php(61): event()
#17 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Controller.php(54): App\Http\Controllers\Auth\RegisteredUserController->store()
#18 /website_path/vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php(43): Illuminate\Routing\Controller->callAction()
#19 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Route.php(260): Illuminate\Routing\ControllerDispatcher->dispatch()
#20 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Route.php(205): Illuminate\Routing\Route->runController()
#21 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Router.php(799): Illuminate\Routing\Route->run()
#22 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(141): Illuminate\Routing\Router->Illuminate\Routing\{closure}()
#23 /website_path/app/Http/Middleware/RedirectIfAuthenticated.php(32): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#24 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): App\Http\Middleware\RedirectIfAuthenticated->handle()
#25 /website_path/vendor/mcamara/laravel-localization/src/Mcamara/LaravelLocalization/Middleware/LaravelLocalizationViewPath.php(22): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#26 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Mcamara\LaravelLocalization\Middleware\LaravelLocalizationViewPath->handle()
#27 /website_path/vendor/mcamara/laravel-localization/src/Mcamara/LaravelLocalization/Middleware/LaravelLocalizationRedirectFilter.php(22): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#28 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Mcamara\LaravelLocalization\Middleware\LaravelLocalizationRedirectFilter->handle()
#29 /website_path/vendor/mcamara/laravel-localization/src/Mcamara/LaravelLocalization/Middleware/LocaleSessionRedirect.php(23): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#30 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Mcamara\LaravelLocalization\Middleware\LocaleSessionRedirect->handle()
#31 /website_path/app/Http/Middleware/IsInstalled.php(22): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#32 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): App\Http\Middleware\IsInstalled->handle()
#33 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Middleware/SubstituteBindings.php(50): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#34 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Routing\Middleware\SubstituteBindings->handle()
#35 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/VerifyCsrfToken.php(78): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#36 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Foundation\Http\Middleware\VerifyCsrfToken->handle()
#37 /website_path/vendor/laravel/framework/src/Illuminate/View/Middleware/ShareErrorsFromSession.php(49): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#38 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\View\Middleware\ShareErrorsFromSession->handle()
#39 /website_path/vendor/laravel/framework/src/Illuminate/Session/Middleware/StartSession.php(121): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#40 /website_path/vendor/laravel/framework/src/Illuminate/Session/Middleware/StartSession.php(64): Illuminate\Session\Middleware\StartSession->handleStatefulRequest()
#41 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Session\Middleware\StartSession->handle()
#42 /website_path/vendor/laravel/framework/src/Illuminate/Cookie/Middleware/AddQueuedCookiesToResponse.php(37): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#43 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse->handle()
#44 /website_path/vendor/laravel/framework/src/Illuminate/Cookie/Middleware/EncryptCookies.php(67): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#45 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Cookie\Middleware\EncryptCookies->handle()
#46 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(116): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#47 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Router.php(800): Illuminate\Pipeline\Pipeline->then()
#48 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Router.php(777): Illuminate\Routing\Router->runRouteWithinStack()
#49 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Router.php(741): Illuminate\Routing\Router->runRoute()
#50 /website_path/vendor/laravel/framework/src/Illuminate/Routing/Router.php(730): Illuminate\Routing\Router->dispatchToRoute()
#51 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php(200): Illuminate\Routing\Router->dispatch()
#52 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(141): Illuminate\Foundation\Http\Kernel->Illuminate\Foundation\Http\{closure}()
#53 /website_path/vendor/barryvdh/laravel-debugbar/src/Middleware/InjectDebugbar.php(59): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#54 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Barryvdh\Debugbar\Middleware\InjectDebugbar->handle()
#55 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php(21): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#56 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ConvertEmptyStringsToNull.php(31): Illuminate\Foundation\Http\Middleware\TransformsRequest->handle()
#57 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull->handle()
#58 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php(21): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#59 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TrimStrings.php(40): Illuminate\Foundation\Http\Middleware\TransformsRequest->handle()
#60 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Foundation\Http\Middleware\TrimStrings->handle()
#61 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ValidatePostSize.php(27): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#62 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Foundation\Http\Middleware\ValidatePostSize->handle()
#63 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/PreventRequestsDuringMaintenance.php(86): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#64 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Foundation\Http\Middleware\PreventRequestsDuringMaintenance->handle()
#65 /website_path/vendor/laravel/framework/src/Illuminate/Http/Middleware/TrustProxies.php(39): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#66 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(180): Illuminate\Http\Middleware\TrustProxies->handle()
#67 /website_path/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(116): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#68 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php(175): Illuminate\Pipeline\Pipeline->then()
#69 /website_path/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php(144): Illuminate\Foundation\Http\Kernel->sendRequestThroughRouter()
#70 /website_path/public/index.php(53): Illuminate\Foundation\Http\Kernel->handle()
#71 {main}
"}