Event Rule condition on custom_fields not working?

[mathieumd]

I have a Webhook launched by an Event Rule with a condition.

When I set this condition, then the webhook is not called:

{
    "and": [
        {
            "attr": "status.value",
            "value": "planned"
        },
        {
            "attr": "primary_ip4.address",
            "negate": true,
            "value": null
        },
        {
            "attr": "custom_fields.vm_link",
            "negate": true,
            "value": null
        }
    ]
}

However, if I remove the custom_fields.vm_link part, then the webhook is called and working correctly!

The VM I use to try this is always the same, and of course it's Custom Field vm_link is null, as shown by it's API page:

Are Custom Fields not available in Event Rule conditions?