Custom Script broken after upgrade to 4.2.9

[ql-network-team]

After upgrading from version 4.0.9 to 4.2.9 I am having issues with some existing custom scripts

An exception occurred: TypeError: Direct assignment to the reverse side of a related set is prohibited. Use site.set() instead.

Traceback (most recent call last):
  File "/opt/netbox-4.2.9/netbox/extras/jobs.py", line 43, in run_script
    script.output = script.run(data, commit)
                    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/netbox-4.2.9/netbox/scripts/FTTP-Provisioning-v6.py", line 52, in run
    new_child_prefix = Prefix(
                       ^^^^^^^
  File "/opt/netbox-4.2.9/netbox/ipam/models/ip.py", line 281, in __init__
    super().__init__(*args, **kwargs)
  File "/opt/netbox/venv/lib/python3.12/site-packages/django/db/models/base.py", line 564, in __init__
    _setattr(self, prop, value)
  File "/opt/netbox/venv/lib/python3.12/site-packages/django/db/models/fields/related_descriptors.py", line 665, in __set__
    raise TypeError(
TypeError: Direct assignment to the reverse side of a related set is prohibited. Use site.set() instead.

Its a long scripts so wont post the full thing. It fails on this line:

new_child_prefix = Prefix(
            prefix=f"{first_prefix.network.format()}{prefix_cidr}",
            tenant=ql_tenant,
            site=ql_site,
            role=network_subnet_role
)

The variable is set from user selection:

site_name = ObjectVar(
        description="Name of the new site",
        model=Site
    )

ql_site = data['site_name']

I've had a play with a few things but this now reaches my Python limit. Does anyone know how to fix this? I'm fairly sure its user error not a Netbox bug but maybe it actually is