Invalid Authorization Header in Dingo

I'm implementing a secure REST Api with Dingo. I was able to return a token after POST username and password. Now I'm trying to access a secure end point. Here is the API code.

$api = app('Dingo\Api\Routing\Router');

$api->version('v1', function ($api){

    $api->post('authenticate', 'App\Http\Controllers\APIController@authenticate');
});


$api->version('v1', ['middleware' => 'api.auth'], function ($api){

    $api->get('drivers', 'App\Http\Controllers\APIController@getDrivers');
});

I'm using postman and when I try to access /drivers, it gives following error.

via Chebli Mohamed