L5.1 command api:cache need twice

When I call `api:cache` first, the file: `bootstrap/cache/routes.php` is

> app('api.router')->setAdapterRoutes(unserialize(base64_decode('YTowOnt9')));

and run `api:routes`

>\>php artisan api:routes
>
> Your application doesn't have any routes.

the http respond is:

>"message": "The version given was unknown or has no registered routes.",
>"status_code": 400,

Then I call `api:cache` command again, it is normal.
So, what is the problem?