Reference a null payload value

Following the documentation for implementing a NOT rule. I need to check and make sure the head_commit in the payload is NOT null.

Webhook is not accepting my NOT rule and I'd like to know if there is a known issue with using NOT or if anyone has successfully implemented a NOT rule.

{
"not":
  {
    "match":
    {
      "type": "value",
      "value": null,
      "parameter":
      {
        "source": "payload",
        "name": "head_commit"
      }
    }
  }
}