phpSpec no beCalled([array:0]) matcher found for null

I'm trying to get into the workfow of using phpSpec to desing my classes. I have stumbled on testing a handle method on one of my event handlers.

My spec:

use App\Order;
use App\Models\Payments\Payment;
use App\Services\Payments\PaymentService;
use App\Events\Payments\AccountPayment;

class RecordPaymentTransactionSpec extends ObjectBehavior
{
    function let(PaymentService $paymentService)
    {
        $this->beConstructedWith($paymentService);
    }

    function it_is_initializable()
    {
        $this->shouldHaveType('App\Handlers\Events\Payments\RecordPaymentTransaction');
    }

    function it_should_record_payment_transaction(AccountPayment $event)
    {
        $this->paymentService->recordPayment($event)->shouldBeCalled();
    }
}

And this my implementation:

class RecordPaymentTransaction {

    public $paymentService;

    /**
     * Create the event handler.
     *
     * RecordPaymentTransaction constructor.
     * @param PaymentService $paymentService
     */
    public function __construct(PaymentService $paymentService)
    {
        $this->paymentService = $paymentService;
    }

    /**
     * Handle the event.
     *
     * @param  AccountPayment  $event
     * @return void
     */
    public function handle(AccountPayment $event)
    {
        $this->paymentService->recordPayment($event);
    }

}

However, I keep getting this error:

 - it should record payment transaction
      no beCalled([array:0]) matcher found for null.

Cannot see what I'm doing wrong here, help please.

via Chebli Mohamed