Issues with create a Stripe Subscription in Spring

I am building a Spring app which uses Stripe for subscription based payments. I am having issues when creating a Subscription, the exact error that I am getting is: org.hibernate.AssertionFailure: non-transient entity has a null id: com.randomprogramming.projectstrics.entity.Account, but that doesn't really make sense since when I open my database(which is PostgreSQL), I can see that the Account has an ID. But when I check the Subscription field on my Account, it's empty. Here is the code which is supposed to create a Subscription and save it in my database:

public Subscription createSubscription(CreateSubscriptionBody createSubscriptionBody, Principal principal) throws Exception {
        logger.info("Started creating a new subscription...");
        // probably not necessary to check both the principal and account but just to be safe
        if (principal == null) throw new Exception("Principal not defined, check if you are logged in.");
        Account customerAccount = accountService.findByUsername(principal.getName());
        if (customerAccount == null) throw new UsernameNotFoundException("Error when searching for user.");

        logger.info("Creating the Customer and PaymentMethod...");
        // Create a Customer object from the passed in Customer ID
        Customer customer = Customer.retrieve(createSubscriptionBody.getCustomerId());
        // Create a PaymentMethod from the passed in Payment Method ID
        PaymentMethod pm = PaymentMethod.retrieve(createSubscriptionBody.getPaymentMethodId());
        // Attach the Customer ID to the Payment Method
        pm.attach(PaymentMethodAttachParams.builder().setCustomer(customer.getId()).build());

        logger.info("Creating CustomerUpdateParams and updating Customer...");
        // I don't even know what this does
        CustomerUpdateParams customerUpdateParams = CustomerUpdateParams
                .builder()
                .setInvoiceSettings(
                        CustomerUpdateParams
                                .InvoiceSettings.builder()
                                .setDefaultPaymentMethod(createSubscriptionBody.getPaymentMethodId())
                                .build()
                )
                .build();
        // and we update the customer with whatever the fuck we did above
        customer.update(customerUpdateParams);

        logger.info("Creating the Subscription params...");
        // Create the subscription params
        SubscriptionCreateParams subCreateParams = SubscriptionCreateParams
                .builder()
                .addItem(
                        SubscriptionCreateParams
                                .Item.builder()
                                .setPrice(PRICE_ID)
                                .build()
                )
                .setCustomer(customer.getId())
                .addAllExpand(Collections.singletonList("latest_invoice.payment_intent"))
                .build();

        logger.info("Creating the subscription...");
        // Create the subscription
        // I'm pretty sure that Subscription.create is async, so be careful with this
        Subscription subscription = Subscription.create(subCreateParams);

        // Save the subscription that was returned into our database
        StripeSubscription stripeSubscription = new StripeSubscription(subscription.getId(),
                subscription.getCurrentPeriodEnd(),
                subscription.getCustomer(),
                // Since we're only going to be charging for one thing, we can use 0 index here
                subscription.getItems().getData().get(0).getPrice().getId(), customerAccount);
        save(stripeSubscription);

        // Save the subscription to the account and save the account into the database
        customerAccount.setSubscription(stripeSubscription);
        accountService.save(customerAccount);

        logger.info("Finished creating the subscription.");
        return subscription;
    }

Upon running the program a few times with my debugger, I noticed that the error happens on the line save(stripeSubscription);. When checking the values with the debugger, everything has an Id, and everything looks fine to me..

Here's the save method:

    public void save(StripeSubscription stripeSubscription) {
        stripeSubscriptionRepository.save(stripeSubscription);
        logger.info("Saved StripeSubscription in the database.");
    }

Here's my StripeSubscription Entity:

@Entity(name = "stripe_subscription")
@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
public class StripeSubscription {
    @Id
    private String id;
    private Long currentPeriodEnd;
    private String customerId;
    private String priceId;

    @JsonIgnore
    @OneToOne(mappedBy = "subscription")
    private Account account;
}

Another thing to note is that the subscription actually gets created, which I can check from my Stripe dashboard.. What am I doing wrong?

Hmm, I tried creating a new Id field in StripeSubscription and saving the Subscription id in another field, and that seems to have fixed the problem..