Laravel 5项目中的NotFoundHttpException

I'm a newbie building a webapp using laravel 5, but got stuck with a NotFoundHttpException. I looked at several similar questions but could not solve the problem.

My routes/web.php file

Route::group(['middleware' => ['web']], function () {

    Route::get('/', function () {
        return view('welcome');
    });

    Route::post('/signup', [
        'uses' => 'UserController@postSignUp',
        'as'=> 'signup'
    ]);
});

app\Http\Controllers\UserController:

class UserController extends Controller {

public function postSignUp(Request $request){
    $email = $request['email'];
    $username = $request['username'];
    $password = bcrypt($request['password']);

    $user = new User();
    $user -> email = $email;
    $user -> username = $username;
    $user -> password = $password;

    $user -> save();

    return redirect() -> back();
}

public function postSignIn(){

}
}

I am sending the request like this:

<form action="{{ route('signup') }}" method="post">

I checked for any typos several times but couldn't find any. Thanks for the help.