Laravel 5.3 Call to a member function groupBy() on null - methods

Question

I'm getting the following error:
FatalErrorException in EloquentVehicle.php line 30: Call to a member
function groupBy() on null
I have the following code:
<?php
namespace App\Project\Frontend\Repo\Vehicle;
use Illuminate\Database\Eloquent\Model;
class EloquentVehicle implements VehicleInterface
{
protected $vehicle;
/**
* EloquentVehicle constructor.
*
* #param Model $vehicle
*/
public function __construct(
Model $vehicle
)
{
$this->$vehicle = $vehicle;
}
/**
* Fetch unique makes
*
* #return mixed
*/
public function fetchMakes()
{
return $this->vehicle->groupBy(array('make'))
->orderBy('make', 'asc')
->get();
}
}
I've checked Illuminate\Database\Eloquent\Model for the method which is obviously not there, but I don't know what I should be adding to my class so that I can use the groupBy method. The laravel docs say the method exists.
UPDATE: Apparently I can't typehint an abstract class. I don't know how else I should be going about using Eloquent to retrieve records. If it helps, below is the code I have for registering the classes to the service container
<?php
namespace App\Providers;
use App\Vehicle;
use App\Project\Frontend\Repo\Vehicle\EloquentVehicle;
use Illuminate\Support\ServiceProvider;
class RepoServiceProvider extends ServiceProvider
{
/**
* Bootstrap the application services.
*
* #return void
*/
public function boot()
{
//
}
/**
* Register the application services.
*
* #return void
*/
public function register()
{
$this->app->bind('App\Project\Frontend\Repo\Vehicle\VehicleInterface', function($app)
{
return new EloquentVehicle(
new Vehicle
);
});
}
}

Answer 1

I just found my mistake and quite literally lay my face in my palms.
This
$this->$vehicle = $vehicle;
should be this
$this->vehicle = $vehicle;