Using XPath to specify trigger in XLSForm

Hi all!
I'm trying to trigger a calculation in an XLSForm, and everything works fine when I specify the trigger using the ${} format, but when I try to use the XPath format, XLSForm Online (v2.x) throws an error when I try to convert the form, saying "Error: Only references to other fields are allowed in the 'trigger' column." Is there a trick to this? Is this kind of specification simply not allowed for triggers?

To provide an example, the trigger works fine when I specify it with ${unit}, but not /data/maize_acq/unit, where unit is the name of the question I'm using as a trigger, and it's nested inside the maize_acq group.


Just bumping this - could this be a bug in the parser and unintended behavior?