Can I create a field transformation using a JSON source key?

I don't seem to be able to set up a field transformation using a Source Key that comes from a JSON event field.

I have events like this:

{
  "time": "2013-06-23T13:55:37+00:00",
  "handler": "UsersController#index"
}

And I'd like to extract "UsersController" and "index" from the "handler" field. I have props.conf configured with KV_MODE=json

I added this transform via the GUI:

[controller_action_transform]
CLEAN_KEYS = 1
MV_ADD = 0
REGEX = (?<controller>.*)#(?<action>.*)
SOURCE_KEY = handler

and this field extraction:

[json]
REPORT-controller_action_extraction = controller_action_transform

however, when I do a query like this:

sourcetype=json | table handler controller action

I do get results for "handler" but don't get anything for "controller" or "action":

| handler               | controller | action |
-----------------------------------------------
| UsersController#index |            |        |

If I change the transform SOURCE_KEY to "_raw" then I do get results for controller & action (though not exactly correct).

Also, I can do an inline "rex" field extraction using the "handler" field and get the correct results. That is, this works just fine:

sourcetype=json | rex field=handler "(?<controller>.*)#(?<action>.*)" | table handler controller action

Am I doing something wrong with the transform? Are JSON-extracted fields not available for use in transforms or something?

(NOTE: The above is just some sample data I created for testing this out. The real logs that I need to use this on have more data and nested keys and so forth, so a workaround that involves not using the extracted JSON fields would be pretty non-ideal.)