To fix it :
/**
* Retrieve a Coupon instance by its ID
*
* @throws HttpException:
* - If the couponId is invalid (the coupon does not exists...)
*
* @see https://stripe.com/docs/api#coupons
*
* @param string $couponId: The coupon ID
*
* @return Coupon
*/
public function retrieveCoupon($couponId)
{
try {
return Coupon::retrieve($couponId);
} catch(\Exception $e) {
return false;
}
}