Using the following style"
I get an error "no visible layers"
It looks like this occurs because the function is getting tied up in the zoom-level code, which only checks for a long or a JSONObject, not a string.
This probably shouldn't be getting caught by the zoom-level code in the first place.
Turns out this was not actually the error I though it was; I forgot to include "property": "FEATURECLA", in the function definition, and got a rather useless error message.
Improving the error message should be all that is necessary here