Stripe client_reference_id: null in checkout.session.completed

I have a stripe pricing table on my front end to render the options available to customers. On my backend I am using Django to handle account creation if the payment was successful. As below this was created following a tutorial on TestDriven.io btw.

if event['type'] == 'checkout.session.completed':
        session = event['data']['object']
        print(session)
        # # Fetch all the required data from session
        client_reference_id = session.get('client_reference_id')
        stripe_customer_id = session.get('customer')
        stripe_subscription_id = session.get('subscription')

        # Get the user and create a new StripeCustomer
        user = CustomUser.objects.get(id=client_reference_id)
        StripeCustomer.objects.create(
            user=user,
            stripeCustomerId=stripe_customer_id,
            stripeSubscriptionId=stripe_subscription_id,
        )

I am getting the following error

raise self.model.DoesNotExist(apps.authentication.models.CustomUser.DoesNotExist: CustomUser matching query does not exist

So after some digging I have found that Stripe is not returning the client_reference_id to my backend, below is a printout of the session variable

"client_reference_id": null,

I believe this is why the account is not being found by Django, but I can't seem to figure out why Stripe isn't sending this information over or how to attach it to the response to my Web hook?