Lumen Authentication: unknown index: provider

I'm using Lumen for the first time and trying to setup token authentication. After following all of the instructions in the lumen documentation, I'm getting the following error when trying to access a page;

ErrorException in AuthManager.php line 152:
Undefined index: provider
in AuthManager.php line 152
at Application->Laravel\Lumen\Concerns\{closure}('8', 'Undefined index: provider', '/home/webdev/public_html/project/vendor/illuminate/auth/AuthManager.php', '152', array('name' => 'api', 'config' => array('driver' => 'token'))) in AuthManager.php line 152
at AuthManager->createTokenDriver('api', array('driver' => 'token')) in AuthManager.php line 92
at AuthManager->resolve('api') in AuthManager.php line 70
at AuthManager->guard(null) in Authenticate.php line 38
at Authenticate->handle(object(Request), object(Closure))
at call_user_func_array(array(object(Authenticate), 'handle'), array(object(Request), object(Closure))) in Pipeline.php line 124
at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
at call_user_func(object(Closure), object(Request)) in Pipeline.php line 32
at Pipeline->Laravel\Lumen\Routing\{closure}(object(Request))
at call_user_func(object(Closure), object(Request)) in Pipeline.php line 103
at Pipeline->then(object(Closure)) in RoutesRequests.php line 621
at Application->sendThroughPipeline(array('App\Http\Middleware\Authenticate'), object(Closure)) in RoutesRequests.php line 382
at Application->dispatch(object(Request)) in RoutesRequests.php line 327
at Application->run(object(Request)) in index.php line 31

The steps I've taken are;

Uncommented the following in 'bootstrap/app.php'

$app->register(App\Providers\AuthServiceProvider::class);

Also added this line to 'bootstrap/app.php'

$app->middleware([
    App\Http\Middleware\Authenticate::class
]);

I've setup the database connection details in '/env', and I've created a user table which contains an 'api_token' field. Here's the User model;

namespace App;

use Illuminate\Auth\Authenticatable;
use Laravel\Lumen\Auth\Authorizable;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Contracts\Auth\Authenticatable as AuthenticatableContract;
use Illuminate\Contracts\Auth\Access\Authorizable as AuthorizableContract;

/**
 * @property int $id
 * @property bool $active
 * @property string $api_token
 * @property string $full_name
 * @property string $email
 * @property string $username
 * @property string $password
 * @property string $avatar
 * @property string $background
 * @property string $date_last_login
 * @property string $facebook_id
 * @property string $facebook_token
 * @property string $twitter_id
 * @property string $twitter_token
 */
class User extends Model implements AuthenticatableContract, AuthorizableContract
{
    use Authenticatable, Authorizable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'full_name', 'email',
    ];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [
        'password',
    ];
}

I have made no changes to the Authentication Middleware, and the AuthServiceProvider also remains exactly the same with no changes.

I've done my best to trace through the lumen code to see where 'provider' is supposed to be set, but I can't seem to find where the issue is.

Can anyone help?