flysystem: the method of read() in ftp not worked, File not found at path:xxx file

My laravel vision is 5.2.*:in my config/flysystem.php ,here are configs:

'disks' => [
            'ftp' => [
            'driver'     => 'ftp',
            'host'       => 'myhost',
            'port'       => 21,
            'username'   => 'myusename',
            'password'   => 'mypassword',
            'root' => '/',
        ],
],

and my route is here:

Route::get('/flysystem', function () {
    Storage::disk('ftp')->put('hi.txt', 'Contents test');
    echo Storage::disk('ftp')->read('hi.txt');
});

it is error and output “FileNotFoundException in Filesystem.php line 381: File not found at path: hi.txt”; My English is very bad ,please help me! thank you.