Constructor on Injected class not getting called Laravel IOC

I am injecting the InvoiceService in the testclass. Test class calling the create method of the invoice service.. Constructor of the injected class not getting called because of this $repo is null create is failing. Any help is appreciated.

$this->app->singleton(
    'App\Models\Subscription\Interfaces\IInvoiceService',
    'App\Models\Subscription\Impl\InvoiceService'
);



class InvoiceService implements  IInvoiceService
{

    protected $repo;


    public function _construct(){

        $this->app = App::getFacadeRoot();

        $this->repo = $this->app['repo'];


    }
    public function Create($array)
    {
    $this->repo->insert($array); // //$this->repo is null as Constructor not getting called
    }
 }


 class TestClass 

 {
    protected $InvoiceService;
    public function _construct(IInvoiceService $InvoiceService){

            $this->InvoiceService =$InvoiceService

    }

    public create($input) {

        this->InvoiceService->create($input);  // This is failing
    }
 }

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