Optional route stop others route with same pattern

The following is my route to define the optional variable, and i will have a middleware to process the optional variable. I not sure why this make the testing page inaccessible and i believe it is interpreted as the invite code. What's the workaround for this?

Route::group(['middleware' => ['friend']], function()
    {
        Route::get('/{invite_code?}', 'IndexController@index')->name('home')->middleware('firsttime'); // 首页
        Route::get('/testing/{invite_code?}', 'IndexController@testing')->name('testing');
    });

via Chebli Mohamed