"Exception occured" trying to update row

See this post for background about the problem I’m trying to solve.

I originally tried to create a single button that would update the last row, with the following configuration:


I originally tried to use the formula ModifyRows(ActiveAfflictions.Last(), Stacks, CurrentValue.Stacks - 1), but unfortunately, ModifyRows does not appear to support CurrentValue. The version with FormulaMap was suggested in another topic here.

Trying to run this button simply causes an error popup with “Exception occurred”, however.

You can see this at https://coda.io/d/FoP-Charsheet_dc0oNBLOu_5/Buttons_su9bw#_luUsu; I’ll try not to delete the offending button as I work on other stuff.