Sub-task 1: Tweaker always reports failure 

A users reports the following:

Hi! I am trying to use the new Tweaker task you added support for in the last release, but I cannot get it to work.

Regardless of what I do, the task always reports a failure. Here is an example of a flow that produces the failure:

{
"hostname": "host.name",
tasks": [
    { "type": "TweakerTask",
      "description": "Consume two cores",
      "mode": "ConsumeCores",
      "tweak": 2
    }
        ]
}

I am almost sure I am following the documentation, but regardless of what values I send in it always reports “FAILURE”. What am I doing wrong?